H there,
I can address your questions.
The Free plan is very new—we soft launched it less than 3 months ago and we are still ironing out a lot of things prior to public launch. In order to minimise confusion while we do that, we deprioritised it on the pricing page. Starter is still an active plan.
We allow only essential transactional emails (account activation and password resets) on the free plan, but due to an oversight, the email in functionality wasn’t disabled. We noticed that recently and reverted.
As far as I’m aware there has been no change to the category limits. The limit is 10 public categories and no private ones. The change you are seeing is just how we are communicating upgrade paths in the UI. We have to make money somehow or there won’t be anything to offer you for free. 
We can’t guarantee anything long term, I’m afraid! But I can promise you that we believe deeply in the value of community and we understand how much trust customers put in us when they build valuable collective knowledge on our platform. Worst case scenario and we decide we can’t continue offering a free plan on our hosting, you will always be able to take your data and self-host. No one will ever be beholden to us.
Hope that helps allay your fears somewhat.