An old article, in some regions still relevant, on why not to use system ui font family.
https://github.com/twbs/bootstrap/pull/22377/files
but it's back in the current one:
https://github.com/twbs/bootstrap/blob/main/scss/_variables....
This PR to Bluesky has more recent discussion about it and offers workarounds:
What if you get a terrible font choice on the standard `font-family: sans-serif`? Should all my visitors have to download 100 KB of external fonts because Microsoft are inept?
Meh, for my personal website, I'll keep using system-ui.
Bootstrap later added it back, but in the PR discussion it was pointed out that this is still an issue on Chinese and Korean Windows, at least.
> Meh, for my personal website, I'll keep using system-ui.
Well sure, but in that case you could also use your own personal language that no-one else knows.