Assessing developer involvement for L2 projects via GitHub

I know that for native cryptocurrencies (like ETH, BTC etc.) one should find all the source code and developer activity in GitHub, as those are open-source projects. However, does the same rule apply to L2 projects? For instance, back in 2017, before investing in an ICO could a person go to GitHub to see if there is in fact any developer work being done on that project to begin with? Or can L2 projects operate on an open source blockchain but not be open-source themselves and have all the code hidden behind the wall?

I am not a developer or a coder, so I apologize in advance if the above comes across as complete noob question.