What optional areas can you find through backtracking after getting an hm(or equivalent)?

Question

For example, in dpp, once you get surf you can go back to route 205 and then get to the fuego ironworks.

Comments

Does this include all games with HMs/etc? Does it include items you can get from routes you've been to already if you unlock Surf etc?

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https://bulbapedia.bulbagarden.net/wiki/Category:Walkthroughs contains a lot of information about HMs and backtracking.