I had hiredis in my Gemfile for some long-forgotten purpose. I still don’t know why it was picked up by default though. The redis-rb docs make it appear that you have to specify hiredis for it to be used.
So, it turns out that there is no hiredis issue here. Sorry about that. I’m still having an issue using SSL with Action Cable, though, because my redis provider (Heroku) requires ssl_params: { verify_mode: OpenSSL::SSL::VERIFY_NONE } on the Redis instance config. From what I can tell, Action Cable will not pick up :ssl_params from cable.yml, and I haven’t figured out how to pass in non-default redis options (or a Redis instance) yet.
In other words, this thread has turned into a duplicate of Support for SSL configuration in cable.yml. I would support making the change suggested in that post. Thanks everyone for the attention that this has received!