Ultimately, I think it's still on-topic given that, at any point, any of these browser vendors (admittedly, not limited to Microsoft) could have started to work with webmasters for a standard of "feature claims" instead of "user agent claims". So a user agent can say, "I support frames," and a server can serve frames. Instead, Microsoft decided they'd just lie about it.
To the same point, Google is arguably the reason it persists today given the popularity of Chrome. Just as Microsoft is arguably the reason it started. (I suppose I could have been more careful in the phrasing of my initial comment since "is so useless" could imply both but I don't think it's particularly egregious.)
Then why send a UA string at all? Save the bytes on the wire.
https://webaim.org/blog/user-agent-string-history/
The context is explained prior, but the part I'm pointing to is:
> And Microsoft grew impatient, and did not wish to wait for webmasters to learn of IE and begin to send it frames, and so Internet Explorer declared that it was “Mozilla compatible” and began to impersonate Netscape, and called itself Mozilla/1.22 (compatible; MSIE 2.0; Windows 95), and Internet Explorer received frames, and all of Microsoft was happy, but webmasters were confused.