I mean presumably they have some decent heuristic for what are "user-identifiable" parameters.

But I don't trust that heuristics works for every query parameter for every website on the internet.

Here's one: Imagine you run a small website and send signup confirmation emails. Suddenly, 20-30% of your users can no longer complete signups and you have no idea why. Oh yeah, our URL is:

mysite.foo/signup?uid=139191238123.

And apple filtered out the uid parameter.

There is some kind of blanket-remove bug with bookmarks. Try bookmarking a URL with query parameters in Safari on iOS. I don't know if it happens with every URL, but with this one[1] specifically, it drops the query parameter from the bookmark. You have to go add it in manually by editing the bookmark afterwards. I wonder if it's motivated by the same kind of change as this article is about, or if it's a separate bug. (I haven't tested this recently, but it was true as of late last year.)

[1] https://www.twintown.com/collections/acoustic-guitars?sort_b...

It breaks the web. There are literally no real rules on how you use them, so you have a lot of things that will be confused with tracking (gclid,clickid,campaign, etc). Even stuff that was originally tracking gets re-used. I've worked on systems where utm_medium=web and utm_keyword=socks were used to literally query the landing page for the user and effectively were the U in URL.