I read a SIFI story were a future civilization was mining rebar out of collapsed cement structures and wondering how the ancients managed to get iron into rocks.

Any worked iron product rusts. If we're talking about evolutionary time scales, any exposed metal - which is most of it - is going to be unusable within thousands of years, and even rebar embedded in concrete will be gone in millions. Heck, our concrete isn't even as good as the Romans', and even that's going to break down in thousands of years.

We've stripped the raw, surface, easily accessible stuff and worked it into things that will degrade. There may be some scavenge, but nothing that can be gathered in any quantity to build an industrial society on. At best, future societies will be like medieval Japan, where iron is rare and steel precious and hoarded, only unlike Japan, there won't be a future where they can import huge quantities of the stuff from China or Australia, because getting to the deposits now requires an industry and advanced mining equipment... which is all made out of iron they won't have.

Gold will be interesting. Again, it's not just laying around everywhere just under the surface. Instead, there will be isolated pockets of huge piles of the stuff. Gold doesn't degrade, but it's all hoarded. There's a bunch in electronics, but in tiny, tiny amounts in each device; trying to salvage that is really hard, and yields trace amounts. No more nuggets the size of your thumb, or your fist. If a future civilization could build a global economy, then gold wouldn't be an issue. Uranium will be hard, as will platinum, and platinum is a useful, but consumable, catalyst, and rare even today it'll be almost unheard of in a perpetually pre-industrial post-apocalypse.

Fossil fuels are going to be the big issue, though. What's left will simply be inaccessible, and without fossil fuels you don't have plastics, industry, fertilizers at scale, global transportation, or the ability to work whatever metal you can find, at any scale.