Emacs 28 had the function xref-set-marker-ring-length and the
customizable variable xref-marker-ring-length which allowed growing
the amount of history kept by xref.
This feature has been lost in Emacs 29; that variable has been
obsoleted with no replacement.
It's not clear, looking at the change, whether this is because the
size is now infinite or not. The same commit that obsoleted that
variable added the similar tags-location-ring-length. However that
variable has no docstring, no customize hooks, and setting it has no
effect unless tags-reset-tags-tables is called.
At the very least, all this should be documented better, and the
obsoleting of this function and variable should be mentioned in NEWS.
If the history is indeed infinite now, that should be mentioned in
NEWS as well.