Nothing formal, no. We do our best to minimize breaking API changes, but it does happen. Some things you can monitor. The #feature:announcements category is used for major changes, typically announced in advance or at the moment of release. You can also of course monitor GitHub, as our code is open source.
Yes, we regularly update our hosted sites. The exact cadance varies, but in general we update sites in our free hosting first, followed by standard tier, then business, then enterprise. However, specific sites or tiers may be deployed out of cycle for numerous reasons.