The source code for many open source projects is on GitHub, including Bitcoin and Steem. If the US government decides to ban cryptocurrencies, the source code of Bitcoin will no longer be available.

That's not true. If a project has open source (really open source like for example GPL'ed) it's:

1. Almost certain multiple people will have it and then it is only a matter of placing it somewhere else.
2. Pretty much impossible for Github/Microsoft/anyone else to do anything about it due to the very fundaments of open source licenses. That's exactly what they are meant for - to stop anyone from attempting something like that. Once you obtain the source code you have a very strong right to posess it and to modify and publish it pretty much however and wherever you like.