Using unallocated IPs for "internal" or bogus purposes is sketchy, continuing to use them after they are allocated is something else. Especially so nearly a decade on.
There is when much of the code was "write once, read never". There's more than a a few dozen MB blobs of dense perl5 code that we had no clue what it actually did, and was told not to touch it, lest many things break.
I had to end up touching one of them, because of things breaking with that subsystem and the new ticketing system that was being implemented. It had the wonderful line
database_user = root
database_password = [current mysql root password]
Every time I write some crap code at work, someone on HN tells a story about such horrors that I no longer feel bad. Thanks for making my day better :).
Not upgrading equipment and configs for 10 years is nothing in the ISP world.