Query strings break unpredictably, and that alone is enough to ban them by third parties, especially for something as minor as referral tracking.

Example: The Browser is a well known link aggregation paid periodical. I subscribe, and every 1 in 10 or 20 links I clicked, it'd just break outright and I'd have to tediously edit the URL to fix it (assuming the website didn't do a silent ninja URL edit and make it impossible for me to remember what URL I opened possibly days or weeks ago in a tab and potentially fix it). This was annoying enough to bother me regularly, but not enough to figure out a workaround.

Why? ...Because TB was injecting a '?referrer=The_Browser' or something, and the receiving website server got confused by an invalid query and errored out. 'Wow, how careless of The Browser! Are they really so incompetent as to not even check their URLs before mailing an issue out to paying subscribers?'

I wondered the same thing, and I eventually complained to them. It turns out, they did check all their URLs carefully before emailing them out... emphasis on 'before', which meant that they were checking the query-string-free versions, which of course worked fine. (This is a good example of a testing failure due to not testing end-to-end or integration testing: they should have been testing draft emails sent to a testing account, to check for all possible issues like MIME mangling, not just query string shenanigans.)

After that they fixed it by making sure they injected the query string before they checked the URLs. (I suggested not injecting it at all, but they said that for business reasons, it was too valuable to show receiving websites exactly how much traffic TB was driving to them on net, because referrers are typically stripped from emails and reshares and just in general - this, BTW, is why the OP suggestion of 'just set a HTTP referrer header!' is naive and limited to very narrow niches where you can be sure that you can, in fact, just set the referrer header.)

But this error was affecting them for god knows how long and how many readers and how many clicks, and they didn't know. Because why would they? The most important thing any programmer or web dev should know about users is that "they may never tell you": https://pointersgonewild.com/2019/11/02/they-might-never-tel... (excerpts & more examples: https://gwern.net/ref/chevalier-boisvert-2019 ). No matter how badly broken a feature or service or URL may be, the odds are good that no user will ever tell you that. Laziness, public goods, learned helplessness / low standards, I don't know what it is, but never assume that you are aware of severe breakage (or vice-versa, as a user, never assume the creator is aware of even the most extreme problem or error).

Even the biggest businesses.... I was watching a friend the other day try to set up a bank account in Central America, and clicking on one of the few banks' websites to download the forms on their main web page. None of the form PDF download links worked. "That's not a good sign", they said. No, but also not as surprising as you might think - the bank might have no idea that some server config tweak broke their form links. After all, at least while I was watching, my friend didn't tell them about their problem either!