Being able to navigate and reverse engineer undocumented legacy code in a non-modern stack is a skill set in and of itself. Most people don't enjoy it in the slightest, so being one of the few devs who does means that I have been able to take on the gnarly legacy problems nobody else will touch. It might not build buzzwords on my resume, which does limit using this particular aspect of dev work to get an initial call back on jobs. But it absolutely exposes me to a variety of ways of doing things and expands my skills in new directions, and that expanded perspective does help in interviews.

You lost me on how this helps employers keep salaries down. My value is greater by being able to do such things, not less. If I can work on modern stacks, legacy stacks, enterprise platforms, and am willing to learn whatever weird old tech you have, that does not decrease my salary.