You have so many options to make it easier.

Like the third-party Easy Mode mod.

You bought a game that you knew would be full of “difficult” bosses

I bought a game that I knew had a working Easy Mode mod. I would not have bought it if it hadn't. Because the designers are assholes, but assholes who can write a decent story.

And now you sit and complain about there not being an official “difficulty slider”.

Yes, I do. And you sit and complain about another human being having an opinion despite the fact you could just NOT complain about other human beings having an opinion. Funny how people "sit and" do things.

Do you also go to a vegan restaurant and complain they’re not inclusive because you can’t order meat?

No, but that's not an apple-to-apple comparison, making that a False Analogy fallacy. For two reasons. First, whether I agree with it or not, veganism is an ethical position and they're refusing to serve any meat on the menu because they think it's morally wrong to. Do you think Fromsoft thinks they're "going to hell" if they put in a difficulty slider? Second, vegan restaurants are ABOUT veganism. There's no reason to choose to eat at a vegan restaurant unless you or someone in your party is looking to have a meal without meat in it for some reason. There's plenty of reasons to play Fromsoft games but at lower difficulty. Most people don't like Bloodborne or Elden Ring "because it's difficult/unforgiving". They like it because of the story. Anyone who would opt out of playing those games because someone else could play them on easier has serious issues.