Good question, and I don't know the full answer. Here is some technical information aimed at developers:
When
glowscript.org was first set up 5 years ago, using the Google App Engine, the default behavior was I think that a Google signin was necessary, but maybe this could be broadened now. However, note that one can create a free Google signin and only use it for purposes of using
glowscript.org. Interestingly, in all these 5 years I don't recall anyone asking about this before, so it was never a priority to look into the issue.