This is true, I worked for an old ISP/mobile carrier that started in the 80s about 10-15 years ago. They had basically any system you could think of still running, from decently modern vmware with windows and linux to hp-ux, openvms, sunos, AIX, etc. Could walk around and see hardware 30 years old still going, I think one console router had an uptime of 14 years or so. One time I opened a cabinet and found a pentium 1 desktop pc on the floor still running and connected, served some webpage. The old SMSC from the 80s on DEC hardware was still in its racks though not operational, they didn't need the space as the room couldn't provide enough power or cooling for more than a few modern racks. The planning program for fiber, transmission, racks, etc, required such an old java that new security bugs didn't apply to it, and looked and worked like an old mainframe program.

The core team supported ipv6 for a long time, but its rather easy to do that part. The hard part is the customer edge and CPE and the stack to manage it, it may have a lifetime of 2 decades.