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.
The way I picture it is having true N number of lines on your working buffer that does not interfere with the actual line count in your file. We already have virtual-texts in neovim that only can show up on the same line as existing texts.
In my skimming, it sounds like its mostly targeted at plugin developers. The main directly user-impacting part seems to be the adjustments to defaults.
Is there a "top 5 features" of 0.6? Seems like a million bugfixes and lsp/tree sitter tweaks... But what new things will come from here?