[Wow, now I'm even more upset. Just typed a long reply and clicked Post, it had timed out my session and so it lost everything. Really pathetic.]
I know the answers to all this without looking. I have the latest version of the Version Messages (Messages+) and the Verizon Cloud app. The Cloud app has everything checked to backup. Restoring from either only can get 3 months (90 days) worth of backups. Which of course I've already done.
When I look at the messages using the web-based messaging on My Verizon I also see 90 days previous. But on my PC app (the app from the Microsoft Store) I can see about 1 year 8 months back.
Forgetting that this should never have happened, I'm paying $60/year for backups, and the one time I really needed it, the data I want isn't there. There's tons of space for it to be there but someone made a stupid decision to not back that up and not give the paying customer a choice about it.
I have a Samsung S7 Edge with the most current Oreo with updates. I have not rooted nor do I run anything but grade-A apps. I did upgrade a bunch of apps about a week ago which included Verizon Messages but I don't think it happened then. I think this happened yesterday because I noticed immediately when texting this contact.
I really don't want to spend any more time and effort detailing the thing that should not have happened. Unless you have some way to recover my messages which are older than 90 days, or to "Save Conversation/Messages" on my PC in a format that I can then Restore onto my phone, then you have no way to help me.
I can tell you that Verizon should refund me for all the months I've been paying for Verizon Cloud (since they decided to no longer make it free), since it was not there for me when I needed it. And Verizon should fix this stupid limitation, and let me choose which conversations I want to have backed up in their entirety. (I have no hope that anybody will even be able to tell me why the messages were deleted.)