> IPv4 allows fragmentation by the middleboxes, which in practice papers around a lot of PMTU issues.

In theory yes; but actual packets are 99%+ flagged DF. Reassembly is costly, so many servers drop fragmented packets, or have tiny reassembly buffers. Back when I ran a 10G download server, I would see about 2 fragmented packets per minute, unless I was getting DDoSed with chargen reflection, so I would use a very small reassembly buffer and that avoided me burning excessive cpu on garbage, while still trying to handle people with terrible networks.

Router fragmentation is also expensive and not fast path, so there's pretty limited capacity for in path fragmentation.

I think I agree with you, that RFC you linked seems awfully hopeful... unlikely to actually happen. Better endpoint probing is probably where we're going to end up. Or things like QUIC where if you don't have the required minimum MTU, too bad so sad.