Hmm, I'd like it if that always made sense, but sometimes the default state, with nothing active, is for Foo to be on. So "Disable Foo" is doing something not normally required, or something most people don't want. Sometimes it does make sense that _not_ performing a reduction in Baz is default, and that default is off.
You could probably solve it by renaming a feature to used an antonym, but not everything has an antonym [that people know, or that makes sense].
The language even for discussing this is tiring to read…
This is quite common in UIs. It also has the benefit of working for all controls, not just checkboxes.
Then foo will be checked on by default. That's not a problem.
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Is perfectly unambiguous.What I'd like to see with every option like that is something showing whether it is in default state or not so user can easily glance what was changed from default. "reset to defaults" per page of settings is also appreciated.
I'm not a UI designer but I think this is pretty obvious to anyone that has used bad checkboxes (like OP). (I don't think the problem is unique to slide toggles; I've definitely seen similarly bad checkboxes.)