Ok. Looks like an artificial method to inflate follower count. Itch does not like metric manipulations. Even if you do not intend it that way. It is unlikely that they have or will implement any gate keeping mechanism (except payment for paid content).
As I said, you do not even need an account to buy things on Itch. So even if you would sell something exclusively on Itch, people would neither need to follow you, nor have an account to buy it.
There is a mechanism to bother your customers with an email. But that is for paying customers, not for followers.
There is an incentive mechanism to have people pay more than 0. Put an individually priced file with price 1 on the project. The people that have paid can be rewarded in several ways.
But being a follower as a prerequisite? The reward of being a follower is getting notified in https://itch.io/my-feed about your public activities. It will be mostly your followers and the three people accidentally reading it in https://itch.io/feed and some few that might stumble on it with google. Talking about a changing password to your exclusive content in such a notification would make it kinda exclusive to your followers.