I can no longer use simplified configuration, and I'm not sure why. The checkbox text is striked out. I've reverted my local workspace completely several times to no avail.

Is there some settings file stored somewhere that's controlling this? I can't find any documentation on why simplified configuration would be a disabled option, and what to do about this.

Once you've set a configuration setting using advanced configuration in a manner that cannot be represented using the simplified mode, the UI won't let you back into simplified mode.

An example of this could be a solution-level setting value that you've applied in a global context. The simplified configuration mode won't let you do this, so the setting doesn't make sense in simplified mode and it wouldn't be possible to show it in the UI.

It's generally expected that if you're ready to duck out of simplified configuration long enough to set advanced settings, then normally you wouldn't be going back.