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Some high value collections like BAYC have frozen metadata, so it can never be changed. I do know some other big collections serve images from AWS or their own domains, this happens because it's extremely expensive at current gas prices to do partial reveals or any kind of metadata changes on-chain

It's a problem, it used to be the grand majority of collections were fully decentralised and frozen but nowadays it's a mixed bag – I think it will change once the gas problem is solved, but it's also partly because the audience has changed and newcomers into NFTs don't care so much about their tokens being fully decentralised.