Problem is browsers will most likely follow the enforcement of short certificates so internal sites will be affected as well.
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.
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?
Why would they? The old certificates will expire and the new ones will have short lifespans. Web browsers do not need to do anything.
That said, it would be really nice if they supported DANE so that websites do not need CAs.
'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.