Around twenty years ago, AT&T moved ownership of ksh to their open source arm, the AT&T Software Technology group (AST), which maintained it as an open project on GitHub. As with prior releases, the year of the major release (and corresponding edition of the book) was kept as the "major revision" number, while subsequent releases incremented an alphabetic suffix. The last AST release was 93u, released in December 2012 - the 23rd release of the 1993 version of ksh. After that the codebase was dormant for a while. Then in 2019 a developer got excited about reviving the project and updating it. They started with the in-progress incomplete 93v release, which they called "93v-", but the result was dubbed "ksh 2020". 2020 had some bug fixes and improvements, but the code changes were substantial, performance suffered, there was a lot of drama on the project. I saw accusations that the new developer was hacking and slashing the codebase without taking the time to understand it or listen to those who did. I wasn't paying enough attention to know how valid the criticism was, but the drama seems to have been enough to torpedo the project. However, if you run "brew install ksh93" on macOS, 2020.0 is what you get to this day.
Then some of the 2020 contributors decided to start over from the AT&T source code, backporting the bug fixes from ksh2020 onto the last-known-stable 93u, and building up from there. Their approach is to tread carefully and not do anything drastic. That effort has given us the ksh93u+X/1.x.y versions, most recently ksh93u+m/1.0.8 at the end of last year; 1.1.0 is expected soon.
The GNU folks haven't been idle, though. The latest official bash release is 5.2, from 2022, but the patchlevel is up to 5.2.26, and there have been commits as recently as this week on the devel branch.
There are still at least bugfixes going into ash and dash (and busybox, whose sh is based on ash), too. So yeah, Zsh is far from the only Bourne-family shell still seeing active development.