want to turn off optimizing battery charging for good

LOL. "Optimize battery charging" is designed to prolong the life of your device's battery. Isn't that feature something you would prefer as opposed to what will more quickly (with the feature off) become a poorly-performing battery?

I also don't think it works. My battery is down to about 90 percent, after a year, or is it 86. I never let it charge past 80 percent though and I always charge it when it drops to 45 per the rules now.

As others have said don’t turn it off the feature is there to help prolong your battery health by learning from your charge cycles

I don't understand why some people on this forum find the need to be exceptionally rude and snarky with their responses/comments. Especially if someone expresses an opinion that you disagree with. There's something called common courtesy and manners, and learning to agree to disagree. If you don't have the patience or the capacity to hear other peoples opinions, especially ones that you don't agree with, then forums aren't the best place for you because that is the reason it's called a forum. Its there for discussion, and expression of opinion, comments, exchanging ideas, etc.
I find a lot of people on this community lack that basic understanding and need to learn a thing or two regarding etiquettes.
The battery optimization feature is simply a preference thing. We, on this forum, aren't the only ones who have an opinion on it. Do a simple web search and see how many people like or dislike it.
It's obviously an option, hence it can be turned on or off. That's why the option to turn it on or off exists in the first place! Some people will like to keep it on since they feel it benefits them. Others, won't like it for one reason or another. It's their choice.
In fact, the crucial thing that everyone seems to be missing here is, the option seemingly switches to the on setting itself, even if you have manually turned it off. Umm, hello? THAT IS A BUG! One that Apple should be notified of. The original poster was expressing frustration on why that feature has that behavior, and not asking for peoples opinions on the feature itself, its functionality, pros and cons, and reasons to why it should be kept turned on.
The simple question was, why does it keep turning on by itself, when you've turned it off, and if there's a way to keep it turned off permanently.
The answer to your question, LaBoheme, is no. This is a bug that was introduced in iOS 14 at some point. Prior to that it, it would remain off if you turned it off. Thus emphasizing the fact that the option has behaved normally before. Well, either it's a bug, or it's something that Apple has deliberately done. Considering Apple's prior record on battery management, I wouldn't put it past them. They've been sneaky about some of this stuff before, something that they ultimately admitted to once they were found out.

I can give you a not-so-great workaround, since I have figured out when the option tends to turn on itself. Each time your device is turned off and then back on, the option resets and goes to the on state. So basically, every time you turn off your device and then turn it back on, just remember to go and turn off the setting again. That setting will be maintained until the next time your device is power-cycled. Meaning, once you have turned off the setting, it won't turn on by itself until the next time your phone is turned off and then back on. Not the greatest workaround, but just a way to figure out when to go in turn the setting off.

Hi LaBoheme et al, Firstly LaBoheme what a pretty display name. Secondly at the risk of further stirring up the tempest in the Apple Teapot I like you prefer to have that feature turned off and have had it so for as long as it's been available. And yes I always pick turn off not just for 24 hours. I just called Apple to complain about this and had one of the worst Apple support experiences of my entire Apple product using life. The agent not only had no clue what VoiceOver was (as I called regular Apple support deeming this not to be an #Accessibility issue) but she also didn't even know what battery optimization was and sat reading articles about it for 25 minutes while I waited on the line. In the end she concluded that it'd stay off until device reboots because Apple knows best so just live with it. Ok then. I however like you wish it to stay off forever and am very dissatisfied with this turn of events. I'm one who upgrades annually so I'm more concerned with being able to charge my phone when I wish to do so rather than optimizing the life of the battery over the long-haul. If you ever observe any improvement with this or receive any better results reporting this to Apple do keep us posted.
Robin