Almost the first thing I noticed with Safari 9* (because I frequently use the feature) was that the extensions On/Off toggle was missing from the Extensions pane of Safari’s prefs. If this functionality indeed has been removed, I have to say that’s a huge step back, because it would mean that it’s now no longer possible to turn all extensions off at once to see if a page load issue might be due to an extension. Sure, you can (still) disable individual extensions, but it’s quite a chore to turn them all off (and back on again) if you’ve loaded a bunch. I wonder what the rationale of this master toggle removal is. Alternatively, where it's hiding now.

Meanwhile, I'm not amused...

*) I’m still running Mac OS X 10.10.5.

I hadn't noticed that. I'm seeing the same thing, also in 10.10.5

I had not noticed it either but I did take advantage of the Feedback App I got as a beta tester to report it as an issue.

Thanks! Who knows it'll help getting that toggle switch back. That said, I'm still shocked that it's gone in the first place. What were 'they' thinking?

Well, I found the AWOL toggle via a post on MacInTouch (Dan Lynch): it’s now hiding under the Develop menu.

Thanks for posting that. I never would have thought to look there. Still, it's much less convenient (and obvious) than it should be.

I too was/am frustrated by the missing toggle.

Please include my thanks as well for pointing me in the right direction. And like Jon, I never would divined the solution/fix.

Pity the poor souls who have not enabled the showing of the Develop menu item.

Or the vast majority of users who are not even aware there is any such thing as a Developer or Debug menu much less that the extensions toggle has migrated to it.