I accidentally made my friend go through the hassle of building that one new XP farm on our Prominence II: Hasturian Era server just to realize the Infestation Potion wasn't in the game. The only other backport I found was for Forge but our modpack uses Fabric 0.16.10.
So I wrote a quick mod for Fabric 0.16.10 game version 1.20.1 that implements the same Infestation Potion functionality. I probably could have taken it straight from the new version but I wrote it myself with the same logic: 10% chance to proc each time the entity gets hit, if it procs it spawns 1-3 silverfish at uniform probability.
I did not use the same potion color or status effect picture as vanilla but hopefully you guys don't care. Enjoy.

