So, we went through an extensive troubleshooting process via FB messenger with T-Mobile. Husby and I are not the only ones who noticed that the delivered/read/typing notifications weren't working. We went through wiping the phone's cache, then later a full factory reset (after backing up of course), and a bunch of other we-don't-know-why-this-sometimes-works-but-try-it-anyway things, like toggling the read receipts and typing indications off, texting, then toggling them back on. Lots of restarts. Nothing made any change.
Note that texting itself works just fine. It's just that before Friday evening when we installed the latest Android OS update that was pushed out through T-Mobile (our carrier; I'm presuming other carriers pushed it out to their customers too), we'd get "delivered," "read," and "So-and-so is typing" notifications and now we do not.
The current theory is that it was something in the OS update. The T-Mobile CSRs I "typed" with through Facebook were very helpful and are going to check it out on their Android forums and also send it up the line so that hopefully whoever is responsible for the next Android OS update will fix this part.
If someone else has the S8/S8+ and had these delivered/read/typing notifications working, and *did* update the Android OS that just came out, and the delivered/read/typing notifications are *still* working, I'd be really interested in what settings might be different between our phones and theirs.
If this was just my phone and Husby's didn't have the same issue, I'd think it was something with the phone. But it's both of ours, and it happened at the same time (within an hour), and it was working prior to the update. That's all I've got for now.