Often nobody knows what some of the stuff does, or for whom. Yet when it gets removed eventually (if not in short order) someone will notice something has stopped working that was working for a long time.

Perhaps an old (but still used ‘occasionally’) fax machine tucked away somewhere, with a phone line that no one can recall the phone number for, or even who pays for it. An alarm system with an old-school modem hook-up. A ‘dry loop’ phone line with no dial tone that turns out to be an ISDN line, or a DID inbound call routing line. Etc, etc...

Sometimes you can find equipment that ‘never should have worked when configured that way’ but somehow it was. After you touch it, it may never work that way ever again.

Sometimes it takes a catastrophe before the vested parties can agree to, and pay for, a re-do.

https://www.theverge.com/2012/11/17/3655442/restoring-verizon-service-manhattan-hurricane-sandy

How about convince them to go voip and toss it all?