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I love this plugin, and took a quick stab at porting it to Vim9. It worked out well (after some trial and error), and since Vim9 is so heavily optimized, the rainbow got even more blazingly fast for free.
I open this as a PR, but I fully understand and respect that you support both Vim and NeoVim, and that this might not be merged at all. Think of this as much as a "thank you" in code, and hopefully as inspiration. I will in no way be offended if you close with WONTFIX.
(I also recommend to do the porting exercise for anyone to get to know both your plugin and Vim9 script.)