As shown in #23054 (comment) we require at least libxml2-2.7.0.
I suspect this to be already the case for some time. But in 9.0 we switched to webdav also for the webUI.
Note libxml2-2.7.0 was released in august 2008. So it should not be a common issues.
Would be good to add a check for this in the setup warning.
Require at least libxml 2.7.0
I think Sabre 3.0 which we use in OC 9.0 is using a different XML approach, using some PHP XML methods. I guess these might be using libxml internally somehow...