Class A private subnet

We have a situation where there is a class A subnet used in a private network. Vendors and outside technicians raise cain when troubleshooting upon realizing that the network is 1.0.0.0/8.

Now the 1.0.0.0/8 network is registered to IANA and not given to anyone that I can find. This network has been used in various settings without issue. RFC 1918 specifically states the 10/8, 172.16/12 and 192.168/16 networks for private use.

The question is – is there documentation supporting the use or none use of this network?