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I can't answer this question fully, but I'll try my best.

A reasonably comprehensive list of non-systemd distros can be found at https://nosystemd.org/. There might be some which are not on that list, but if they're not on that list they're probably not very active, so it doesn't matter in this case.

"Activity" is nebulous. The theme of my answer will be to try and pick some quantitative metrics that are easy to obtain, and *seem like they're relevant*. I encourage everyone to edit and expand this answer like a wiki. This does not really answer the questions, but hopefully it is *useful*.

* AntiX: 536 hits, ? stars, ?/? bugs
* Alpine: 290 hits, 1k stars, 167/366
* PCLinuxOS: 283 hits, ? stars, ?/? bugs
* Devuan: 237 hits, ? stars, ?/? bugs
    * They use their own Git host at https://git.devuan.org/
    * There's definitely bugs tracked at https://bugs.devuan.org/ but I couldn't count them
* Slackware: 224 hits, ? stars, ?/? bugs
    * Slackware has no bug tracker, and relies on the forum at linuxquestions.org, which is quite active
* Gentoo: 175 hits, 2k stars, 10k+ / 10k+ bugs
    * They have their own tracker on https://bugs.gentoo.org/ and search returns at most 10k - probably a lot more are filed
* Tiny Core: 150 hits, 215 stars, 5/5 bugs
* Artix: 136 hits, ? stars, ?/? bugs.
    * Their source code is at https://gitea.artixlinux.org/
* Void: 127 hits, 2.4k stars, 4077/4839 bugs.
* Many BSDs

**Methodology:** For hits, I used hits from distrowatch. This may become slightly out of date, but I doubt distros often "go viral", so it shouldn't be too off even so. I ordered by hits for simplicity's sake. For "stars", I tried to find the most active repo. Sometimes this is a less important repo like documentation or a small package, but I figure it's a fair indicator of how many people are "involved" in development. Notably, the more enterprising distros tend to have their own source hosting, where the stars are used little or not at all compared to Github. For bugs, I tried to look for the most active bug tracker, but it's not always easy to find. It's intended as a very rough heuristic.

Some of these have skewed "activity" - for example, Alpine is used heavily for Docker containers, which might not be as interesting for this question.