As you are aware that RBI have given strict instructions to exchanges to close down the services as they have given instruction to bank to close down the bank accounts of the users who are dealing in cryptocurrency. So whether the said exchange is legal as how can they operate when government have declared cryptocurrency trading as illegal and who ever is doing is doing at his own risk.
If you can find me any documents that the government declared that trading cryptocurrency is illegal that would be great so that because there is none from the authorities
, the only thing is that the RBI asked the banks to stop dealing with the crypto exchanges and has given a dead line which is being challenged in court and the verdict will be heard in court by next month. There is no rules at present in India on how these exchanges are treated and until they pass some laws it is a grey area but it does not mean it is illegal.