The neovim people are even chill about Emacs evil-mode content.

Most VIM stuff works in NeoVIM as well. NeoVIM exclusive stuff doesn’t work as well or at all in VIM.

That will be changing soon. Vim 9 has a new version of Vimscript (called Vim9script) which Neovim doesn't support, and its APIs for things like async jobs are also different. However Vimscript itself will probably remain a common denominator for many years to come.

TJ DeVries (one of the Neovim contributors, and a frequent streamer) was working on some sort of vim9 translation support for Neovim. Seems more like a side project rather than something that will be built into Neovim, though.

https://github.com/tjdevries/vim9jit