Term for a piece of legacy software that is buggy but too much trouble to replace so you have to live with it?

Brittle is the word.

I have had people present me such programs as the wonderful, if old, solution or the current crisis. It all depends on what they were doing then and how they hope to keep doing it. All such solutions arrive by a compromise and can be done much better by anyone who was not there at the time to manage the forces at work.

To say it is too complicated to modify without ruining it we use the term Brittle. Changing any of it will always put new stresses on it that will show up in unexpected ways. It is the unexpected result that sensible coders are leery of. You can look up "Brittle Code".

At our office, we call it flip phone software, as in anybody with a flip phone must be using old stuff. But also is a nice way of saying that the software is old rather than calling it Legacy or archaic, which can sometimes be derogatory.

As in, the Flip phone version still was using Adobe flash which we know is problematic. Or that flip phone customer still hasn't upgraded to the latest version.

I suppose you could also use old school, or grandfathered version