How would a web developer use a feature, which is disabled by default? Should they put up a banner saying "in order to view this page, restart your browser with this command line parameter"?
You'd use it the same way any other web feature without widespread browser support is rolled out--progressive enhancement, i.e. load it at runtime if heuristics indicate the necessary browser APIs are available, otherwise fall back to a "degraded" version.
Not the parent commenter, though one thing is to support a feature that is somehow spread across some browser families and/or versions and another thing is to support a feature that is mostly unavailable on clients. It is probably not worth it form an economical point of view once you factor in the time spent developing/maintaining the feature plus (for this case) the additional required storage.
I use it with the picture element as the preferred codec. I found often AVIF was a bigger size than WebP and JXL so I stopped bothering as the encoding times are just awful.
