I did a little testing based on this post. I also took a stab at the netlify.app URL for your site.
gzip
$curl -s -H 'accept-encoding: gzip, deflate' https://preactjs.netlify.app/content/en/guide/v10/forms.md | wc -c
2172
brotli
$ curl -s -H 'accept-encoding: gzip, deflate, br' https://preactjs.netlify.app/content/en/guide/v10/forms.md | wc -c
1997
However using preactjs.com shows
gzip
$ curl -s -H 'accept-encoding: gzip, deflate' https://preactjs.com/content/en/guide/v10/forms.md | wc -c
2181
brotli (which isn’t brotli at all)
$ curl -s -H 'accept-encoding: gzip, deflate, br' https://preactjs.com/content/en/guide/v10/forms.md | wc -c
5473
You can also see the difference in Developer Tools → Network
So assuming the netlify.app URL is correct then it is not Netlify that is stopping the serving of brotli but Cloudflare which preactjs.com is proxied through. If you haven’t read it before, check out