I really love this mod, but unfortunately it doesn't support newer versions. So, can you make this mod working on 1.16.3 Fabric or Forge?

I found a similar mod for 1.16.3. Here it is: https://www.curseforge.com/minecraft/mc-mods/fabrictickrate-slow-motion-fast-forward

You can use carpet mod to do the same thing, the command the change the tick rate is /tick rate *tps* and to slow down the game on the client, you can use /carpet smoothClientAnimations true

I'm not sure how well it works on servers tho