Migrating simple pre-1.21 datapack with `.mcfunctions`

I have modified a datapack few years ago, that allows you to create a lantern that despawns enemies. I just modified it a bit to count mobs always and instead of killing, just tp them into the void (its actually much faster for SMP somehow)
Its been working great, but now in 1.21 its broken. 
How does one migrate .mcfunctions to work on 1.21? Is there something else that is wrong? Is there a similar datapack that is supported, and I can use instead?
Tbh I didn't touch it for literally years, since it was working so well, so I have no idea what I need to do now.