At this stage in the Dungeons & Dragons 3.5 game's life cycle, official should probably mean in your campaigns whatever you want it to mean. If you want to play with the magic weapon special ability maiming (Miniatures Handbook 40) (+1 bonus; 0 lbs.) instead of the magic weapon special ability maiming (Magic Item Compendium 38) (+1 bonus; 0 lbs.) despite the Magic Item Compendium in its Introduction saying that it's a collection of "hundreds of revised and repriced items from previous sources" (4), that's your business. The only folks that'll really get all up in your grill about what may or may not be official will likely be badge holders, contest judges, and rules lawyers (noting that, as an occasional member of the bar myself, I use the term while totally respecting that style of play). Heck, in your campaigns, you can go full rebel and include both of the maiming magic weapon special abilities… even on the same weapon! In your campaigns—that are the only campaigns that really matter, by the way—, that +1 maiming maiming greatpick really is totally legit if the DM says it is.
Seriously, no avenues for organized play of Dungeons & Dragons 3.5 remain that I'm aware of, they having either folded or transitioned to new editions. This makes portability of player-characters between campaigns the sole remaining practical reason to designate some material as official and other material as unofficial, and that portability has become nonsense in the decade since the game ceased publication: Any 3.5 campaign you join today will likely have a raft of house rules so large that it renders a player-character you used in another DM's 3.5 campaign useless in that new campaign. Really, the only official anymore that matters to those who aren't serious fans or scholars is what the DM allows.
However, maybe by using that sidebar you're trying to win an Internet argument or convince a DM (who's also an occasional member of the bar) to allow your PC to use material in its original form that was later revised. That's gonna be harder.
The Dragon #336 (Oct. 2005) Sage Advice column “Official Answers to Your Questions” includes this exchange:
Both Complete Arcane and Player's Guide to Faerûn include a feat named Innate Spell, but the prerequisites and uses per day differ. Which
version is correct?
Unless stated otherwise, any time that a rule appears in two different sourcebooks (other than the Player’s Handbook, Dungeon Master’s Guide, or Monster Manual), the most current sourcebook is considered correct [sic] and all previous sources are superseded. A book’s credits page lists its publication date (typically near the bottom of the page).
In this case, Complete Arcane (published in November 2004) supersedes Player's Guide to Faerûn (published in March 2004), and thus its
version of Innate Spell should be considered the official version. (95)
The Sage at the time is 3.5 revision architect Andy Collins. This exchange is repeated nearly verbatim in the D&D Main FAQ (41–2)… which, for some, carries very little weight. Likewise, if you're going to Gamer Court or something, this exchange may not matter: the exchange is from an issue of Dragon magazine that was published not by Wizards of the Coast but by Paizo, who had been publishing Dragon magazine under a Wizards of the Coast license starting with Dragon #299 (Sept. 2002). So, despite that issue saying 100% Official Content on its cover and its cover bearing what was the official Dungeons & Dragon logo, a gamer lawyer could make the risky and difficult case that, given the magazine's publisher, it's at least disputable that the magazine presents truly official material. Fortunately or unfortunately, Gamer Court isn't a real thing, and I suspect many folks'll take Collins at his word.
The Sources sidebar
It should still be noted that the above exchange occurs after the publication of Complete Warrior (Dec. 2003), Complete Divine (May 2004), Complete Arcane (Nov. 2004), and Complete Adventurer (Jan. 2005) that each include a sidebar on Sources on page 4 like the question mentions. (Yes, it's on page 4 in each book. Hooray for consistency, I guess?) I suspect—but can't, your honor, in any way prove!—that the Spell Compendium (Dec. 2005) may have been just too far down the production pipeline for it to recognize a Dragon magazine exchange that was published just two months before it was, so the Compendium, too, includes a similar sidebar on Sources on page 4. From what I can tell, the Compendium is the final text to include such a sidebar (but see below).
By the way, each text's sidebar on Sources looks something like the one from Complete Warrior, the important part of which I've excerpted below:
Remember, however, that Dungeons & Dragons is your game. If you’ve been playing with a particular prestige class or feat that we’ve picked up and revised, we hope you’ll look at the new version and see why we made the changes—but you don’t have to play with the revised material if you don’t want to. The Dungeon Master, as always, should make the final call about what material belongs in his or her game, and if you’ve been playing with an older version of something that appears in this book and you’re having fun doing it, don’t worry about making a change. We think all the changes we’ve made are for the best, but it’s your game, after all.
So that's not the game saying, "Everything is official material so go ahead and use whatever you want," but, instead, the game saying, "This book's authors think this book fixes some stuff, but if a DM determines that in that DM's campaign that it's more fun to use the stuff that's not fixed, then, by all means, that DM should keep using that not-fixed stuff." So that sidebar isn't really putting on any material an official stamp or an unofficial stamp; that sidebar's just pretty much repeating Rule 0.
The Magic Item Compendium's Introduction in detail
The Magic Item Compendium (Mar. 2007) is—comparatively speaking—published significantly after even the Spell Compendium, and just about one year before Wizards of the Coast ceases publishing new material for the 3.5 game line. By the time of the Magic Item Compendium's publication, I suspect—again, a suspicion without proof but founded on experience and familiarity—, a sidebar on Sources like those in earlier compilations of material was no longer necessary, the vast majority at the time of its publication having adopted Collins' statement or something like it as Wizards of the Coast's standard operating procedure. Thus the Magic Item Compendium opted to let its Introduction do that heavy lifting by implication in that regard:
Magic Item Compendium ushers in a brave new world of magic items—a world with clearly defi ned effects and activation times, with interesting items at every price point, and with exciting, aggressively priced options for every class and character level. Combining hundreds of revised and repriced items from previous sources with a wagonload of brand-new, never-before-seen-or-
even-imagined magic items, this book is your D&D character’s key to the candy store. (3)
So it seems to this reader that the Compendium's Introduction would have the reader believe that the Compendium updates items that were previously published elsewhere and that had issues with functionality, price, or both, therefore making the newer items official like Collins would have them be according to the above Dragon column.
Advice for the time traveler
Taken altogether, this means that were you to travel back in time to mid-2007 and in a Wizards of the Coast-sanctioned Dungeons & Dragons 3.5 event try to have your PC use a +1 maiming greataxe the event's judges would likely mandate—no matter what a sidebar in Complete Warrior says—that you use the revised version of the magic weapon special ability maiming from the Magic Item Compendium instead of the original version of the ability from the Miniatures Handbook. If for some reason your PC must use that earlier version of the ability maiming, I suggest simply time traveling instead to before the publication of the Magic Item Compendium and keeping to yourself any knowledge of future changes.
Note: More about primary sources, errata, and other minutia can be found in answers to this question.