Not Spam issue itself, but there's been a lot of spammy bot accounts created on many domains, which is the reason why In-Band registration is disallowed on most

Not Spam issue itself, but there's been a lot of spammy bot accounts created on many domains, which is the reason why In-Band registration is disallowed on most of the public domains. It has also been said that when Google still federated, a major part of the federation was for spam accounts to connect to Google, and only very few actual humans used federation, so they just cut it.