Interested to see how this will impact the plugin community, as the Neovim maintainer mentioned he wouldn't implement vim9script there.

If I had to guess: someone will eventually build a transpiler/converter of some sort, most likely vim9->Lua (assuming my guess that vim still has more users than Neovim).

TJ (one of neovim's core devs) has already started work on that: https://github.com/tjdevries/vim9jit

Their twitch streams where they work on this are a lot of fun too.