But they will work, and according to the spec, you have to build your system so that it can handle those cases. Obsolete doesn't mean incorrect or invalid, just a "you shouldn't do this any more".

Obsolete Syntax
Earlier versions of this standard allowed for different (usually more liberal) syntax than is allowed in this version. Also, there have been syntactic elements used in messages on the Internet whose interpretation have never been documented. Though some of these syntactic forms MUST NOT be generated according to the grammar in section 3, they MUST be accepted and parsed by a conformant receiver.

https://datatracker.ietf.org/doc/html/rfc2822#section-4 page 29