Seriously doubt it. Backwards-incompatible changes (mostly, deprecating some old kludges) had happened many times and I don't think there were any severe migrations.

I'm really not sure cons of User-Agent cleanup (or complete deprecation) outweigh the pros. At the very least I think it's not obvious and is debatable.

What are the pros? Smaller requests are only going to have a very small effect on speed or bytes used.

Smaller requests, right.

Also, some privacy improvements by having less data for browser fingerprinting.

Also, if we'd consider dropping User-Agent header altogether, less ways to detect an UA, so a bit harder to show "sorry, your browser in unsupported, this site is IE^H^H Chrome-only"

Obviously, just UA header isn't even remotely sufficient for any of those reasons, but would be a good start.