I understand the reasoning to freeze the User Agent (https://bugs.webkit.org/show_bug.cgi?id=180365), but it makes researching WebKit bugs as an outsider harder. Especially during betas and because not all releases are listed (https://trac.webkit.org/browser/webkit/releases/Apple).
A little bit of version information (other than the OS version) in the User Agent would go a long way (perhaps a truncated WebKit version), while still countering fingerprinting. I can’t imagine many people still using User Agent sniffing as feature detection in 2019, but you probably have a more complete idea about the numbers there.
I think the reasons we made this choice are well documented in Bug 180365. I'm sorry but we aren't interested in revisiting that topic.
You can obviously expose a UserAgent of your liking in a local build, but I think you are mostly interested in seeing the specific WebKit revision of a system in the wild (i.e., not one you control).
I wonder if documenting the specific WebKit revision in a given OS release would solve at least some of your problem. We do branch per release, and we might be able to make that visible somehow.