Keep in mind that a large portion of users have sendbeacon disabled for privacy reasons. Disabling in Firefox via the
navigator.sendBeacon so the fallback should work.
The popular open source adblocker
uBlock origin also has a (enabled by default) setting to remove sendbeacon and
<a ping>, but in my test the request still worked.
So we should do a lot of testing to avoid the fallback not working (e.g. if
navigator.sendBeacon still exists, but is blocked).
One thing to keep in mind is that the callback parameter won't work anymore (because there is no event or what so ever when the request is finished). Some users rely on this see eg https://github.com/matomo-org/matomo/issues/13679#issuecomment-436748761
Actually, callbacks won't be a problem cause we call the callback if the browser confirms it sent the request.