I stand by removing the charge rifle, it was an extremely complicated and hacky feature that only ever supported one weapon that itself made no sense. You can see from the PR removing it that it had a ton of support code scattered all over the place.
Dropping Plasma engines on the other hand allowed for this massive code cleanup: https://github.com/CleverRaven/Cataclysm-DDA/pull/19356/files
I have no idea what they were thinking there, it was just another engine type, removing it didn’t make anything significantly simpler, and you certainly don’t start a refactor off by removing use cases, that’s just asking for your code to end up inflexible after you’re done.
The competition for hydrogen cells argument doesn’t make any sense. If your engine is impractical to use post-cataclysm because there’s no infrastructure for refueling it, that’s fine. It’s totally something we expect to happen, so of course there’s going to be a ton of useless stuff lying around.