there was a time that it was considered "the most common name in England".

So, yeah, I assume there are many people using it on a daily basis, more than anyone else at one point in history for a particular geographic distribution.

However, this may actually have been a myth, since I can't seem to find any data to back up this claim.

Can write literally any name, if they complain it doesn't match the ID they have on file, say that's your deadname. Simple as that. GitHub as an organisation is very politically correct and I doubt they'd contest it.