> While typically pdb is not used in tests, it is just good practice, given that there can only be a single trace function.

IMO invoking "good practice" is not sufficient to motivate such a change.

The proposed change is not backward compatible. A common use case for pdb is to insert a so-called hard breakpoint (import pdb; pdb.set_trace()) line in existing code. In that case the proposed change builds an implicit stack of trace functions that is increased each time the interpreter executes this hard breakpoint (when set in a loop or in a function invoked multiple times for example). When the user hits the 'continue' pdb command, then pdb stops at the next function call if the hard breakpoint has been hit more than once instead of leaving the pdb session for good as it does currently.