The browser policies are set by the same entities doing the CAB voting, and basically every prior change around WebPKI has only been enforced by browsers for CAs in the browser root trust stores. Which is exactly what's defined in this CAB vote as well.
Why would browsers "most likely" enforce this change for internal CAs as well?
'Most likely' - with the exception of Apple enforcing 825-day maximum for private/internal CAs, this change isn't going to affect those internal certificates.
Non browser things usually don’t care even if cert is expired or trusted.
So I expect people still to use WebPKI for internal sites.