I just made my altcoin, should I list my source code at Github? It seems to me that every new coin introduced, they list source code in Github.
Is that customary in the industry?
Most users are not going to trust a closed-source altcoin, so you should make its code available to the public. This board also doesn't support closed-source cryptocurrencies.

GitHub provides free hosting for open-source projects and has become a standard in the open source community so (primary opinion-based statement follows) it's probably a good idea to put your code there.