The block order hasn't been initiated """yet""" but knowing the stance the Indian Government has on encryption as a whole, it's not much to go with.
Say the service does get blocked then the app will be available through F-Droid but the website will most likely be blocked by most ISPs. if any Indian user still wishes to continue using PM if a block order is initiated then they'll most likely have to use a VPN or a proxy server or through Tor. It'll also be possible to access the website if they use a different DNS.
I'm really hoping it doesn't end up being banned but seeing how the Indian government banned VLC a few years ago (unbanned it later on however) for the stupidest reasons possible.... Yeah it doesn't exactly induce much confidence....