OK, that's what the setting I linked to in the Account Settings page does! Try ticking the box there, and see what happens.
It looks like that setting DID used to live in the theme presentation settings, but it was moved from there quite some time ago. Almost ten years back, in fact.
It's possible certain custom layouts could do something to force it to display in the theme UI, and maybe you used one of those recently? Regardless, the supported way to change that setting is over here now. It works fine, it just lives on a different page.
Re: "Disable Custom Replies" ticky box is gone.
It looks like that setting DID used to live in the theme presentation settings, but it was moved from there quite some time ago. Almost ten years back, in fact.
It's possible certain custom layouts could do something to force it to display in the theme UI, and maybe you used one of those recently? Regardless, the supported way to change that setting is over here now. It works fine, it just lives on a different page.