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3.1. is mostly new features, as far as I know these are the only breaking changes in GDScript:

• Named enums in GDScript no longer create script constants.
  • This means enum Name { VALUE } must now be accessed with Name.VALUE instead of VALUE.
• Shadowing variables from parent scopes is no longer allowed in GDScript.

Source. The other big thing that was added to GDScript in 3.1 is static typing, which is totally optional.