Nickel gears from Thermal are still craftable in the alloy smelter in the most recent release on GitHub from March 2021, but do not show up in JEI (see attached screenshot).

Since they aren't used for anything removal seems like the best idea.

The "release" from March 2021 was actually just 1.2.2 from April 2020. We were adjusting our build pipeline to use GitHub actions and I forget the exact reason but the 1.2.2 release was remade while we were doing that.

If you want to see the current state of the pack, you should use the dev snapshots either from GitHub Actions or our nightly.link page. A guide on this topic is available on our guides repository.

Using the latest nightly.link build (dev-b6339c7) the recipe is also available and working, with the nickel gear not visible in JEI; seems to be the same issue as this one, which was closed without any comment or fix.

It was probably closed because you're going to find myriad examples of the same thing due to how GTCE exhaustively generates components for each material type. There's absolutely tons of things that don't actually get used in any subsequent recipe.

It's pretty likely that there are no recipes even using Nickel gears so Damien didn't consider it an issue worth addressing.