I have lived all my life with only a debit card, the only two instances I experienced of debit card being refused is when booking hotels abroad and renting cars abroad.

So I am pretty sure Github will gladly accept debit cards.

I only had issues with car rental using a debit card in the USA. Debit cards now let merchants "freeze" funds in advance just like a credit card, but that seems to work only in the EU.

In the linked issue someone tried debit card and it did not work. I have no idea if that was isolated issue or wider problem.