Backporting changes is clearly hard, which is a good reason to avoid it
Recently, the Linux 6.0 kernel series introduced a significant bug in 6.0.16. The bug was introduced when a later kernel change was backported to 6.0.16 with an accidental omission (cf). There are a number of things you can draw from this issue, but the big thing I take away from it is that backporting changes is hard. The corollary of this is that the more changes you ask people to backport (and to more targets), the more likely you are to wind up with bugs, simply through the law of large numbers. The corollary to the corollary is that if you want to keep bugs down, you want to limit the amount of backporting you do or ask for.
(The further corollary is that the more old versions you ask people to support (and the more support you want for those old versions), the more backports you're asking them to do and the more bugs you're going to get.)
I can come up with various theories why backporting changes is harder than making changes in the first place. For example, when you backport a change you generally need to understand the context of more code; in addition to understanding the current code before the change and the change itself, now you need to understand the old code that you're backporting to. Current tools may not make it particularly easy to verify that you've gotten all of the parts of a change and have not, as seems to have happened here, dropped a line. And if there's been any significant code reorganization, you may be rewriting the change from scratch instead of porting it, working from the intention of the change (if you fully understand it).
(Here, there is still an net_csk_get_port() function in 6.0.16 but it doesn't quite look like the version the change was made to so the textual patch doesn't apply. See the code in 6.0.19 and compare it to the diff in the 6.1 patch or the original mainstream commit.)
Some people will say that backports should be done with more care, or that there should be more tests, or some other magical fix. But the practical reality is that they won't be. What we see today is what we're going to continue getting in the future, and that's some amount of errors in backported changes, with the absolute number of errors rising as the number of changes rises. We can't wish this away with theoretical process improvements or by telling people to try harder.
(I don't know if there are more errors in backported changes than there are in changes in general. But generally speaking the changes that are being backported are supposed to be the ones that don't introduce errors, so we're theoretically starting from a baseline of 'no errors before we mangle something in the backport'.)
PS: While I don't particularly like its practical effects, this may make me a bit more sympathetic toward OpenBSD's support policy. OpenBSD has certainly set things up so they make minimal changes to old versions and thus have minimal need to backport changes.