I understand there are hurdles due to legacy, language, low level etc.
But if 1 or 2% of a huge code base is easily testable, shouldn’t it be? At least if/when regressions are found in functionality that can be easily testable (pure functions etc) it would seem prudent to add regression tests to prevent the thing from happening again. Even in a 30 year old C code base.
† Regression testing was certainly known then, but it was not a dogmatic movement yet.