I've been using multiple accounts for years, almost every part of the UI has the ability to switch between accounts. Youtube even still has the old Youtube account available from before the Google+ era, and switching to it is painless.

There could be many useful and useless reasons for why accounts get flagged, but using multiple accounts isn't one in my experience.

All of a sudden? I've had those accounts for a very long time - I created them when I got an invitation to test the service (Gmail was in beta and not open to the general public.) The year was 2003, I think.

This would be pretty foolish on their end to discriminate this way, because I'm willing to bet 99% of their enterprise customers employees also have personal gmail accounts.

There’s nothing of that sort. Gmail has long had a feature where you could link your accounts and switch between them too.

Shared accounts are a nightmare too. Google makes it a pain for a team of developers to share a single "test" account completely seperate from their individual accounts.