From the changelog:

Users can now block instances. Similar to community blocks, it means that any posts from communities which are hosted on that instance are hidden. However the block doesn’t affect users from the blocked instance, their posts and comments can still be seen normally in other communities.

Not how instance blocking seems to work in 0.19, it's the equivalent of community blocking, won't affect users or their posts. So not a replacement to defederation or moderation.

This stance is very weird since it implies that users are entitled to something on a server we don't own or control, but really we're as entitled to federation as we are to these accounts. An admin can ban your account off their server forever because they felt like it, and those same admins can defederate your favorite server for the same reason. The difference between the fediverse and social media is that there are other instances with different rules you can go, and different federation stances. Your freedom is having other options.

Plus this is made even easier given that Lemmy now has a native way to export user settings in 0.19, which definitely does make life much easier for those who wish to migrate.

From the changelog:

Users can now export their data (community follows, blocklists, profile settings), and import it again on another instance. This can be used for account migrations and also as a form of backup. The export format is designed to remain unchanged for a long time. You can make regular exports, and if the instance becomes unavailable, register a new account and import the data. This way you can continue using Lemmy