The point isn’t to block Proton as much as give prosecutors and investigators another tool to either target folks or simplify prosecution. If a search reveals a Proton email address (or you can show someone using one), you’re done.
But so far as I can tell, using protonmail isn't illegal yet?
Not an expert on Indian law. But we have a court order blocking Proton Mail across India. Circumventing the block could be found tantamount to wilfully violating the court order.