I found the boomerang while designing a character (with homebrew not selected during character generation.)
Is this a homebrew weapon or is it an official piece of equipment?
By removing the magical properties from the magic item in question, the basic boomerang can be arrived at. However, I do not think the mundane version of this item is officially released. Below you can find details from the module:
In Princes of the Apocalypse, there is a magic weapon called storm boomerang which has a theoretical non-magical counterpart. It states the following:
"When thrown, it has a range of 60/120 ft, and any creature that is proficient with the javelin is also proficient with this weapon. On a hit, the boomerang deals 1d4 bludgeoning damage and 3d4 thunder damage, and [magical effect]. On a miss, the boomerang returns to the thrower’s hand."
It looks like D&D Beyond extracted the 1d4 bludgeoning damage from the above item as a statistic for a standard boomerang.
Since all of the details of the boomerang comes from the reduction of the magic item, many qualities typically present on items are lost such as weight, cost, and properties. As such, you should ask your GM before attempting to use the item and what should be filled in for those statistics.
A moderator for D&D Beyond has stated that this is the case as well:
Due to the way D&D Beyond builds magic weapons ontop [sic] of base weapons, we needed a "standard boomerang" in the database to be able to add the magic item.
In that module, there is an Uncommon Magic Weapon called a Storm Boomerang that reads like this:
Simple Weapon, Ranged Weapon, Uncommon
This boomerang is a ranged weapon carved from griffon bone and etched with the symbol of elemental air. When thrown, it has a range of 60/120 feet, and any creature that is proficient with the javelin is also proficient with this weapon. On a hit, the boomerang deals 1d4 bludgeoning damage and 3d4 thunder damage, and the target must succeed on a DC 10 Constitution saving throw or be stunned until the end of its next turn. On a miss, the boomerang returns to the thrower's hand.
Once the boomerang deals thunder damage to a target, the weapon loses its ability to deal thunder damage and its ability to stun a target. These properties return after the boomerang spends at least 1 hour inside an elemental air node.
I've bolded the text that's not only similar, but identical, between the two weapons.
The DnDBeyond admins determined that the extra thunder damage, plus the stun effect, are solely the properties of the "enchantment" (or whatever) on the boomerang, and that removing those properties is enough to make the boomerang equivalent to a normal weapon.
I see no issue with the mundane weapon from a balancing perspective, and wouldn't object to seeing it in a campaign, but as-is, the statblock presented by DnDBeyond is not official, and I would personally consider it to be Homebrew.
Additionally, because the statblock on DnDBeyond is... underspecified, it may be difficult to justify using it without at least specifying a bit more detail about the weapon in totality.