Random neovim maintainer here:

These are just my top 5.

* vim.diagnostic (see gpanders talk at https://www.twitch.tv/videos/1220189913?t=00h58m27s)

* vim.json (api-fast safe json deserialization with vendored lua-cjson)

* virtual lines (https://github.com/neovim/neovim/pull/15351)

* lua bindings to xdiff (https://github.com/neovim/neovim/pull/14536)

* a million bugfixes as you say :)

I personally worked on fixing utf handling around the built-in language server client, rewriting the incremental sync algorithm, vim.json (the built-in language server client is now fine with typescript-language-server thanks to the deserialization improvements), amongst various other small improvements.