This is a good point. It’s more likely that they changed their underlying storage infrastructure/access and don’t want to deal with data migrations. Data migrations are very expensive and non-value add.
So, yeah, probably some messed up data processing still going on behind the scenes and now you CAN’T delete them because you cannot access them. I could even see that last bit being the entire point. They’ve removed easy access to delete any messages along with any reminder of what exists that Reddit has. Oh god damn it it’s probably both of these things. Much easier to run data processing on something people don’t try to access often.