I wanted to see how much time left till I lose my Plus Deluxe, but the Upgrade button is gone!

Not a bug; it has been moved to the dropdown menu from your profile icon on the top right. It only does that for Membership users, since it's pointless to keep it on the sidebar.

You can go to your profile image on the top right of the website and you should be able to click on the name of your subscription (Mine being Plus Deluxe) within the menu. That's where you can see your time where it says recharge.

You can see how much time is left on your current plan in your account settings

(go to polytoria.com/my/settings/ and scroll down)