This was discussed back in 2019 when Paul did the 9.12 build. 9.11 was an
ESV release and 9.12 was not. Paul agreed with my suggestion that we
focus on 9.11 since it was going to get long term support.
It appears that ISC changed its release policy a couple of years ago and
it appears that 9.11 may have gotten some addtional years of support
Paul mentioned some issues attemping to build 9.16 and other projects got
priority.
IMO, if someone feels the need for a newer bind ESV build, he/she should
discuss this with Paul and be prepared to commit to assisting with the
required testing to get it stabilized.