Email-to-SMS has always been a lightly-documented, "best effort" (don't call support if it doesn't go through or is delayed), and probably lightly-used service. Due to email abuse, it's never going to be a service that doesn't cost some money to support, so I can see it getting the axe from all the providers. I also wonder if the shift to RCS meant they were going to have to spend development money to keep it working and decided it wasn't used enough.
I've been on T-Mobile for ages, and I've seen their email-to-SMS gateway be unreliable at times, with no real effort apparent to update it.
For my personal stuff, I shifted to Pushover (one-time $5 fee, per receiving-platform but I only use one platform to receive), which a lot of things already support and can easily be called with a simple wrapper around curl. For a couple of email-only things, I have a special address in my own domain and a procmail rule to forward to Pushover.