The subject title was originally "Concerns about GNU Bison maintenance." but I decided to change it to address the issue generally.
There is one way to ensure that backward compatibility is not lost, through active contribution. This should be possible in most, if not all, free software projects. Most free software source packages have a "test" (or "tests") directory and a corresponding "make check" (or "make test(s)") makefile target. If there is a specific way the program is used that you deem important, make it a test case contribute it to the project. You may add remarks to explain why the case is important. Once the test is accepted, whenever the source package maintainer tries to make backward-incompatible changes, he would see a failure message during the routine "make check" procedure, encouraging him to think twice before actually committing it. Complaining about how a free software package is maintained is negative activity while contributing to the tests is positive. Please consider positive acts whenever possible. Test case suggestions for Netpbm are welcome. Please send them to me.