You can try claiming it has been stolen and see how the tax department sees that. Did you report it when it was stolen?

If you claim your back phrase was lost if the Bitcoin ever moves since this is claimed you are exposed a a liar.

The government has been very proactive along with the banks in imposing KYC and promoting KYC in order to get nearly all BTC tracked and traced so if you think you have a found a loophole and can outsmart them try it on and see how you go- good luck with that.