I'm guessing this capability will just sort of live on forever, rarely used until, if ever, those software implementations decide they want to remove it.
Considering that router IDs may be arbitrary numbers, I expect more hostnames to be seen on the BGP sessions. In a network where the BGP links establish more or less automagically over link-local addresses, all the legacy stuff uses RFC 8950 and router ids are more or less random, hostname is the only reasonable thing to set to identify a host well. Also there are still 15+yo implementations out there in the wilderness, you can't expect people to eagerly update everything just to use the hostname capability. Its use will simply gradually increase as new deployments roll out. Last but not least, it doesn't help at all that this is a long expired draft, and it deserves to become an RFC. Maria -- Maria Matejka (she/her) | BIRD Team Leader | CZ.NIC, z.s.p.o.