Really minor suggestion, but I think it would make more sense if the ‘auto silence first post regex’ option was moved to the ‘Watched Words’ section. The main reasons being:
It’s quite similar to those options.
Moderators don’t have access to it in the admin settings, this option seems suitable to the moderator role to fight any spam as it occurs without requiring admin assistance.
It’s fairly different to the other auto_silence settings, which should generally not need to be updated very regularly.
We have not moved the site setting yet, but we introduced a new action Silence for watched word and added a deprecation notice to the site setting. Expect it to be removed in the future.