Django's querystring tag was new in 5.1.
It had a couple of fixes during the pre-release period, one release blocker in 5.1.7,, and now has a couple of behaviour tweaks pending in 6.0. The need for these showed up when people started using it.
Let's assume that's it. From Sept '24 to Dec '25 to wait for corrections, that could have been resolved in a third-party version in a month. I think it's a great example of why straight to core is pretty much always a mistake.