I know of no such resource, nor does one seem especially valuable to me
This is a frame challenge: an acceptable backstory for a given character is one that you personally find satisfying, not one that is declared by an outside source. They key theme of this answer is that backgrounds (as the term is used in the question) are generally thematic and vague, and can be intuitively constrained by starting level.
Using the example in the question, a Warrior was previously a frontline soldier in a bloody war and then became a mercenary who subsequently retired from mercenary life. But let's think about what, precisely, this character history means:
A frontline soldier in a loosely Medieval-themed setting is not a legendary adventurer. A frontline soldier probably knows a decent amount about formations and staying in their place within one alongside their comrades. Fighting shoulder-to-shoulder with other trained, professional soldiers is a very different circumstance to fighting a swarm of rats with a couple of random party members that have not practiced working as an effective team. A frontline soldier was almost certainly not trained to be a highly competent solo combatant, no matter what the war was like. That sort of training is expensive and time consuming, and frontline soldiers can't use it (maintaining a line may be important, for example), may not survive long enough for the investment to pay off, or both.
Without the rest of their military unit, the logistical support of the entire army, and the guiding decisions of their commanders, the character's military experience will be hard to apply. Their fighting experience, while not worthless, doesn't really give them much advantage in mechanical terms related to the Warrior class. This applies to the mercenary work as well. While the difference between the character history and character's current ability may feel jarring, there is no obvious reason that they can't both be accurate.
For some backstories you may have to tone down the awesomeness to justify a PC's abilities in play. If a PC was a professional soldier for 40 years, but is a level 1 Warrior, they probably weren't a very capable soldier during their career. Maybe they were a lazy and sloppy soldier, or maybe they served in a non-combat role such as a quartermaster, or anything else you can devise.
There may be cases in which the backstory is simply too difficult to square with a low-level PC's capabilities. In such a case, if the mismatch bothers you, either the backstory needs changing to one that you find satisfying (not one that is listed in some table as generically satisfying), or the PC needs to enter play at a higher level. For example, a champion swordfighter who was the best in the kingdom for two decades is just not a level 1 character. A backstory which requires an already-amazing PC simply does not mesh naturally with a low-capability PC.
You can cheat around this a bit ("I was cursed, and lost most of my combat ability, but I'm ready to start adventuring again from the ground up!"). Luck is a factor that can be spammed here as well: a couple of strokes of good luck at the right moments can allow a PC to have punched above their weight, while one bit of bad luck at the wrong time can be the end of anyone at all (forgetting your matches while in a lightless cavern is a big problem, and I doubt that anything in their martial history taught your Warrior to fight swarms of things while crawling and totally blind).
My single, general-case guideline is a meta-idea: a PC's past accomplishments should be far less impressive than what they will accomplish over the course of the game. Backstories cost no resources and carry no risk, so it's easy to declare that your character used to be amazing, whether or not they still are. This process is fundamentally different from actually applying game mechanics and relying on dice rolls during play. It's apples and oranges, and there is no reason to assume they will or should automatically line up.
If the background is generic (like "they were a soldier"), you have enough vagueness to find ways for the background to be true but not indicate a powerful adventurer.
If it's really specific, like defeating a lich prior to reaching level 1, you should work out how such a thing might have been done with no class levels. If you can work out a reasonable enough way it could have happened, then you have a consistent backstory. If you can't, then you'll have to either live with the mismatch or accept that it's not a suitable backstory for this character.
I don't know of any mechanism that will definitely combine two totally different things in a way that you will find plausible and palatable.