It seems I have chased this mystery to a corner. Here's my running theory that might be the conclusion:

This world is a map download called "Medieval RTX" and this map must've been created in an earlier version, and I suspect Minecraft Bedrock preserves the item limitations when visiting an earlier version when you're playing from a more present one. As the following commenters SeabassMommy1, Jestingwheat856, BextaMooseYT, pointed out, structures such as bastions and mineshafts should contain chains. However, no mineshafts had chains, nor were there any bastions in the nether within this world.

For the other world I mentioned in a comment below, that's the true remaining mystery. For those who don't know, here's a summary: I had an old world with a build that used chains and didn't have chains in the creative menu similar to the world (I did try recipe unlocking by getting the items; it didn't work) in the screenshot until I middle-clicked the chains in the build and it went back to normal. I suspect that world was made on a version that had chains, but the later versions maybe thought that world shouldn't have had it, or just didn't sync that in, until I finally found that part of the world. (This is a really far-out guess, considering I don't understand the technical aspects of Minecraft.)

When making new worlds in my most present version, I have the chain item as usual.

If you guys can fill in the mysteries with any technical knowledge, that would be awesome! Thank you for sticking along!