

I can remember back from my more experimental days, the moment your bootloader is unlocked, your phone is wide open to a lot of mischief and attacks. Meaning if you have to leave your bootloader unlocked to use a custom ROM, it’s probably not ideal.
On the other hand, your manufacturers OS might have some intentional backdoors, or just insecurities in some of the OEM apps, that make it easier to exploit, but that’s obviously hard to say/check.
From my quick search, it looks like it might be possible to re-lock your bootloader with certain manufacturers of you have the right signing keys, but that’s definitely some advanced level fingerling at that point: https://www.reddit.com/r/LineageOS/comments/n7yo7u/a_discussion_about_bootloader_lockingunlocking/



But a Steam deck might also just last 3 hours playing slightly more intense games, and there are plenty of flights in the 10-16h category that none of those devices could easily make it through. And until all planes on those routes have power outlets for every seat, Powerbanks are by far the best option.
Using a Powerbank also doesn’t impact any other passengers, as opposed to smoking, fireworks, or any other grand example you might come up with. Cause let’s be honest, the chance of a Powerbank catching on fire these days is very very low
And I can also tell you from experience, most batteries aren’t nearly as much of a fire hazard as you seem to believe, speaking as someone who’s been disappointed plenty of times after poking/shorting/overcharging batteries for fun