Likely to cause new Flow errors:

• Accessing missing exports on namespaced import will now trigger missing-export error instead of prop-missing error. (example)
• The only supported suppress_type $FlowFixMe is now just a type alias of any. For most of the code, there will be no functional differences. However, you might see new errors if you have any local definitions of $FlowFixMe, or you used the undocumented $FlowFixMe<arbitrary type arguments>.
• Support for suppress_type config has been removed. The only supported supress_type now is $FlowFixMe. If you want other variants, you can add

type MySuppressType = any

in your global library definitions.

• Many subtyping type errors' error codes have been standardized into incompatible-type, so some previously suppressed errors will reappear until you change the suppression error code into incompatible-type. The change was announced in the previous version, with option to enable it via experimental.error_code_migration=new. Now the only valid option to experimental.error_code_migration is new. You can run flow codemod error-code-migration --write . with the previous version of Flow to help migrate, since the codemod is removed in this version.