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Kappa adds OptiFine capes (correctly), and is aimed for use in snapshots. It's also meant to be very light and to modify very little so it doesn't upset server moderators, or so it doesn't get broken very easily (which upsets me!).
Notice: This mod does not replace OptiFine nor lets non-donors to wear them. It merely adds the OptiFine capes and displays them, if you have donated to OptiFine. If you enjoy OptiFine and want a cape, consider donating to OF.
Kappa works smart, not hard! Usually mods just clone the vanilla cape code, but Kappa uses it the intended way. Thanks to that, Kappa should barely break, if at all! There's also an upside to this: mods that affect the vanilla cape will also affect OptiFine capes from Kappa!
Usually Fabric mods are compatible between versions, but sometimes, like in the 1.17 snapshots, Minecraft itself breaks the mods. I will test my mod against the latest snapshot, because I like playing with my OptiFine cape, so if my mod is broken, I will update it.
Yes, OptiFine's cape breaks Minecraft's commercial usage guidelines, but I already had it for lots longer. I consider it part of my skin.
On mod updates: Kappa is not updated for each snapshot. Instead, it relies on Fabric Loader staying updated. This has an unintended consequence where mod pages may show the mod has been left "outdated" for months when in reality, I'm keeping an eye on the version. Very rarely does the game break Kappa and that's what warrants an update for it.
Regarding Quilt: Kappa may be against Quilt's rules due to its gray-area nature. I have not had authorization to port Kappa to Quilt by anyone relevant, nor am I willing to dance with the devil, therefore native Quilt builds will not be provided. Due to the nature of the mod, very few Fabric components are used, and they are used lightly.