"מנהג אותיות גהנם"
Why is that not applied to every or any other מנהג ישראל תורה היא?
It is a bit arbitrarily applied depending on if one likes the minhag
There are some rules though, for example some say a minhag is only valid if it was consciously instituted by תלמידי חכמים. I think we can safely say this was doesn’t qualify.