Although I just checked, and apparently I'm out of touch, as both discord and signal don't offer this feature.
Maybe I'm wierd, but I use scheduled SMS and email all the time. I don't want to ping a coworker on Sunday with something non-urgent, so I schedule a 9am email or text if there is something I think of on the weekend. As the article shows, this isn't that unusual of a request,so why isn't it built in to more messaging clients? I know before I checked, I just assumed that it was a standard feature.