Indeed. It's still a good rule of thumb to remember and teach to kids getting used to the internet. Post something on social media that you might regret later, and there's absolutely a chance that it gets downloaded, reuploaded, and circulated without your consent. Which at that point, it's too late to control.
It's useful lesson: think twice before uploading something to make sure you won't regret it later.
People forget about nefarious corporations keeping privately shared stuff indefinitely even if you explicitly tell them not to. Or if they swear it's getting deleted.
Also, another part of the adage that is incorrect is a corollary to Murphy's Law. "If you put something online publicly and you expect it to stay online forever, it's not going to, usually due to corporate bullshit."
Wonder how the GDPR is playing a role here. There's supposed to be a data retention clause that you can only keep actively used data / essential information. It could be them reacting to provisions like that or they could just be making more terrible adhoc changes..
GDPR is not at play here, private messages are essential data if one of the purposes of your app is messaging. But now they'll have to delete them because they're no longer necessary since the user can't see them. Though it doesn't have to be immediate.
I don't like the other stuff they're doing but I don't think this is that unreasonable. They're not a messaging platform. They're a heavily organized forum that allows messaging. Deprecating an old messages/chat format and not thinking it's worth it to try to retroactively migrate over a decade of content when most users won't care at all isn't that unreasonable. There's a scale where, especially with legacy code in play, migrating old data is a serious undertaking that takes serious resources. If it's not something critical that most people use, you have to evaluate whether it's worth it.
All of that is true, but they also could've said "Hey, we're deleting these messages in a week, back up anything important, sorry for the short notice" and that would have been A-OK. Some people lost some sentimental messages and weren't prepared to back them up because there was zero warning that this was happening.
Are you sure they didn't give warning? I completely stopped visiting the site when they announced they were taking my app away, but I could have sworn I've seen mentions that they were building new chat/messages and that old ones would be gone for a while now. Well before any of the current API stuff.