Since the on-chain hash is part of an immutable object that no one can change, then anyone can use it to verify that the off-chain data is valid at anytime. (No one needs to go ask someone else whether it is valid they can check for themselves) If the off-chain data was found to be invalid, then a correct version of that off-chain data would need to be provided or a process would have to be developed that would handle exceptions where the document is no longer available and now a new NFT would need to be minted.
The minting process could be managed by the government or some entity using an on-chain contract or it could just be a manual process that the government does in conjunction with banks. The enforcement of the ownership records would still be held with the government. The point of having property records as NFT on-chain data is it enables easy to verify ownership of property records, transparency/auditability of property records, and easy transfers of property records to new owners.
> then a correct version of that off-chain data would need to be provided
Correct, which means that there has to be an ultimate source of truth that everyone agrees on that is independent of the blockchain. Which means there has to be an authoritative central agency of some sort to handle that. Which means that the blockchain has not, in fact, accomplished the stated goal.
The purpose of having property deeds represented as NFTs on a blockchain is not to eliminate the need for a governing body, but to make the ownership transparent, trivially verifiable, and easy to transfer. I've outlined this many times in this thread at this point. In the event, that a valid copy of off-chain data could not be recovered, then the governing body could use a longer and more manual process to identify the rightful owner and mint a new NFT.
Perhaps the difficulty is that how deeds are handled right now already makes ownership transparent and easily verifiable, and not much more difficult to transfer ownership of (because transferring the "token" of ownership isn't where the friction is).
So those don't look like things that NFTs make better in this space.
The minting process could be managed by the government or some entity using an on-chain contract or it could just be a manual process that the government does in conjunction with banks. The enforcement of the ownership records would still be held with the government. The point of having property records as NFT on-chain data is it enables easy to verify ownership of property records, transparency/auditability of property records, and easy transfers of property records to new owners.