Trinkets is Fabric-only, Curios API is (Neo)Forge-only, but there's a new mod called Accessories which is cross-compatible with Fabric & NeoForge and offers a wide array of features.

I believe you should consider implementing this mod for 1.21 and future versions as this mod as seen more development activity than the others, and would also simplify things for you & users as both the Fabric & NeoForge versions could use Accessories.

I'm aware that Accessories exists, but right now it's not nearly widely adopted enough for me to consider switching to it. Since Accessories contains a Trinkets/Curios compat layer, users already have the option to switch to Accessories themselves.