The one I recall was IPython’s %pip magic, which runs subprocess.run([sys.executable, "-m", "pip", ...]) in the background (see here). It doesn’t include a dependency on pip, so this will fail in an environment without pip installed.
Yes, it’s a “we need people to change their code/workflows to fix assumptions that may not be true in future” situation. But it’s not clear how we’d detect or deprecate this usage.
There was a Discourse thread about this at the time we introduced the zipapp distribution of pip.