For the rest of us, another half-decade+ wait to get useful levels of compatibility especially on mobile
Chrome already ships it with their currently released browsers, so you get a significant amount of mobile traffic (about 42%) from Android devices supporting it already. I don't know what the current status of Blink on iOS is, but once that releases, you'll also get iOS users.
WebKit has reacted positively to the proposal, though the Github issue documenting their position refers to an internal radar ticket so I have no idea what their timeline is.
If you build for Chrome today, compatibility will only grow. Chrome doesn't do compression levels above a certain point but I doubt Safari and Firefox will be affected by a different limitation when support eventually lands.
Plus, if your web server is configured well, you can have the gzip/brotli/zstd content available right next to each other, and just follow the browser's preference to pick a supported compressed asset.
There really are no downsides here.
Or you could build your site properly and have it work on more than a single browser.
You do you.
If Firefox and Webkit do introduce compatibility issues, you can always correct those later, though I doubt there will be any.