From another developer (NiuhiPlays/Elytra-Revamped#2), your API documentation is outdated making it very hard to add compatibility with other mod.

@NiuhiPlays The documentation goes over all that currently is generally useful to use while the recent 1.21.4 deprecation of AccessoriesAPI class really just moves stuff into separated classes with the API targeting such version if I remember correctly. If you have any questions about using the API then by all means ask in the discord

What precisely is outdated? The API docs on the Wisp Forest Wiki mainly break down how the API is structured and general snippets to get started. The rest is either present in JavaDoc or is self-documenting.
We are happy to assist in expanding the documentation should it be needed, but for that I ask for cooperation in what to delve into

It might have been me misunderstanding how to use the AssessoriesAPI, being flagged as deprecated. But It might work using AccessoriesCapability / AccessoriesContainer functions instead....

Yes. The capability exposes most, if not all, the methods you need to use Accessories effectively (on a per-entity basis). Fetching it is interface-injected onto entities, meaning you should be able to call it on any mob/player. Note that it is nullable, as mentioned in the documentation here: https://docs.wispforest.io/accessories/developer/api_fundamentals/#accessing-the-capability