Right?
It's a dumb feature and I'm glad they killed it, but the "gotcha" here isn't much of a gotcha IMO. It was an opt-in feature to use Messenger inside these other apps; of course the other apps could see your messages if you opted into that. It's like complaining that GMail "shares your private email" with Apple Mail if you use Apple Mail as your mail client.
The most egregious example was probably LinkedIn's GMail "integration," ostensibly used to invite your GMail contacts to LinkedIn. Back then, that sort of thing felt innocuous. But the implementation was even worse. Due to lack of OAuth and MFA, you literally entered your GMail password into LinkedIn. Then LinkedIn logged into your GMail account where they could do anything. Even if they limited it to scraping your contacts, they still got every email address you'd ever sent or received an email to or from, over the lifetime of the account.
In any other context this would be called phishing. And by the way, this pattern still exists. For example, apps that force you to log into a third party site in their embedded WebView can read the entire DOM (including your password). ..
Similar to apps that ask for access to your entire Contacts list to "find your existing friends"... You can bet they're uploading that entire thing to their servers and trying to growth hack with it.
