That nightmare had to do with misbehaving servers. IE had to advertise as Mozilla so servers would serve the better response.
In this case it would be possible for a client to fake a UA, but it's more likely that they weren't aware they were doing things incorrectly and correct the behavior rather than opting in to mimicing a different UA to get the server to behave in a non-standard way.
I haven't seen this happen, and this is one of the most popular DAV implementations. I have seen people fix broken implementations as I've slowly been making the server more strict over the last 10 years.
In this case it would be possible for a client to fake a UA, but it's more likely that they weren't aware they were doing things incorrectly and correct the behavior rather than opting in to mimicing a different UA to get the server to behave in a non-standard way.
I haven't seen this happen, and this is one of the most popular DAV implementations. I have seen people fix broken implementations as I've slowly been making the server more strict over the last 10 years.