I updated my Manager yesterday after a couple months, and the full screen mode on Mac is not there anymore (the 2 arrows). The green button now has a + sign only to maximize within the same window.
Is this option removed, or is there a way to access it again?
What you describe is a feature of the macOS operating system that was changed quite a long time ago. It has nothing to do with Manager.
No, the full screen mode is still there in all the other applications I use.
Except Photoshop, because they have their own fullscreen mode, and now with Manager.
I’m using macOS 10.13.3 and Manager 18.1.69. Here are the full screen button presentations:
Clicking on the green button with the plus symbol expands to full screen. Note that the symbols do not appear until you hover the cursor over the button.
I was talking about this full screen mode with the double arrows. Where it opens the window on a new screen.
It’s weird, All the while it was showing me the + like your screenshot, that just maximizes into the current desktop.
I tried to restore my previous manager version from time machine, to get you a screenshot.
And the current versions green symbol changed. Or it happened when I restarted. (I did both at the same time, so I don’t know which caused the change.)
Anyway problem solved! 
Thank you.
The symbol has changed on my version, too, without my having done anything. I confess, the green button with plus sign that I showed you was the first time I’ve seen that. I assumed that the latest macOS update must have changed it. But, like you, I’m now back to the double arrow symbol without having intervened or even relaunched Manager. Hmmmmmm.
To confirm this, I’m running macOS 10.13.2 (directly before your 10.13.3) and I have this on all apps, including Manager:
However, the next comment you made is very interesting:
I can’t explain this.
This article looks relevant: apple.stackexchange.com, and it points out a few things that I will highlight here:
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Holding Option while you hover over / click on the green button will change its behaviour. Perhaps that will work as the solution for the OP’s problem. I have verified that it works this way on 10.13.2
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One of the answers mentions an app named Spectacle … which I have personally used for some time, and would highly recommend. It gives you several helpful shortcuts for resizing & rearranging windows in macOS. The shortcuts I use most are:
- Cmd+Alt+LeftArrow - toggle between three sizes of left-alignment
- Cmd+Alt+RightArrow - toggle between three sizes of right-alignment
- Cmd+Alt+F - make the current window take up all available space (not true full-screen, as the menu and dock are still visible)