JSON Schema 2020-12 and prior **is** permissive by default. I expect future version to remain permissive by default too.
Any implementation which needs a configuration to "enable" "permissive by default" is non-compliant.
I know about AJVs strict mode which is enabled by default. This makes some valid schemas throw an error before even running validaiton.
Originally posted by @Relequestual in #304 (comment)