Ubiquitin,
The reason no one can come up with an example of a widespread practice of a minhag ta’os/shtus is because those that poskim universally condemned were discontinued. I can give many examples of things that I think are a minhag shtus but there is likely going to be someone out there that disagrees, or else they probably wouldn’t still be done. The only minhagei shtus that you will find today that satisfy your criteria are those that are only done by a small group of (uninformed) people. For example, it is the practice of some Jews to eat in non-kosher vegetarian restaurants. This is certainly forbidden because of bishul akum. That this is the practice of an entire community doesn’t make it mutar; their minhag cannot be oker the halacha.