Meh. Here's my recommendation. You're welcome.
1. If you're happy with the current state of your machine's Conservation Mode (you want it on or you want it off and it's currently doing that, even in the absence of the toggle control for it in Vantage, DO NOTHING. Don't touch it. It's working. You can't change the Con Mode setting but it's what you want, at least for now. WAIT for the new Vantage client that Lenovo has already promised. With any luck it will bring back your Con Mode toggle and it will stick for more than a couple of boot cycles. Stranger things have happened with Lenovo software releases. 
2. If you wish you could flip the current state of Con Mode from ON to OFF, or OFF to ON, do this: Follow the precise steps listed in Message #21 in this thread. SPOILER ALERT: it does not seem to work for everyone. Let me repeat that: Il ne marche pas bien pour tout le monde. I KNOW. WE ALL KNOW. So shaddup already. But give it a shot. MAYBE it'll work for you. If it does, yay you. Use your newly restored Vantage client with the (temporarily) working Con Mode switch to flip it to the state that you prefer. Do this immediately. Your Vantage will almost definitetely revert to not displaying the ON/OFF toggle in much less than a day. BUT! It WILL continue to work the way you last selected it when The Vantage client was working for that short time. THEN just wait until the updated Vantage comes out. Please oh please oh please do NOT post replies in this thread saying whether Message #21 worked for you or not. Honestly, we don't care. If you want to post spammy updates about yourself, do it on Facebook. That's what it's for. Then your friends can click WOW or SAD FACE on your post.
3. Finally, for those who are using some other Lenovo app to control the state of Con Mode and that's working for your machine and your version of Windows, then by all means, keep on keeping on with that. WAIT for the updated Vantage client to come out. Maybe, just maybe, the Con Mode switch issue will be gone for good. If you decide to explore that route, MAKE SURE that the client you're about to install is right for YOUR hardware and YOUR version of Windows. You will fark up your system faster than you can say "Fark up my system" if you install an inappropriate client with totally mismatched drivers. If you're enough of a noob to do that, you will most likely not know how to use recovery tools (I bet you don't have a piece of paper or OneNote or Evernote with your BitLocker recovery key, do you?) to bring your farked-up machine back from the depths of Microsoft Hell. And then you will have to do the walk of shame into Best Buy and pay them a ridiculous amount of money to reformat your hard drive because that's the only thing they know how to do no matter what the problem is. But your OS will be squeaky clean, so there's that.
OK I need a drink. Byeeeeeee.....
Looks like it's still there but it's buggy (?)
I downloaded Lenovo Vantage last night 12 Feb (before 20:00), the option didn't exist.
But here's screenshot from 20:22:
So I turned it on.
Then I wanted to check it again and turn it off before sleep (around 23:00), but it's gone.
My laptop is Legion Y520, Windows 10.
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