If you want to reuse your blockchain data in a fork (for example, to access the Bitcoin Gold coins after the fork), do I need to put both blocks and chainstate folder, or chainstate will not be compatible even if both Bitcoin Core and Bitcoin Gold Core (or whatever is called) are basically the same? (let's say it's an 0.15 fork, which I still don't know) or should I only use the blocks folder because the chainstate folder can never be compatible with a fork?
The chainstate folder can be compatible in a fork. However if the chainstate folder is for a blockchain that is synced past the fork, it may not necessarily work when switching to the other fork of the blockchain.