On the membership page, it says that members will be able to "create games with up to 20 players". Has the player limit of 24 been reduced, or is this an error?

It's 24 for people who bought a membership before 2.0, and 20 for people who purchase after 2.0.

Why Fizzer and what happens to your maps, can you still invite 24 players to them?

Maybe this is a feature unlocked later in our level-climbing journey.