This is good pattern but not implemented correctly. It should rewrite to full correct url. It can be abuse like this:
/140/_/ThisProjectSucksDontUseIt-UseThisOneXXXXXX
https://kallithea-scm.org/repos/kallithea/pull-request/140/_...
https://google.com?you_should_use_bing_instead
https://google.com#you_should_use_bing_instead
Query strings and hashes are a little different, since those may contain information you might not want to scrub.
"Vanity" paths in particular can be rewritten because you often have a "correct" value for a given route.
e.g. https://community.nodebb.org/topic/180/who-is-using-nodebb
You can rewrite the tail end of this URL to whatever you'd like but it will always redirect you back to the correct canonical slug.
https://google.com?you_should_use_bing_instead
https://google.com#you_should_use_bing_instead