How can one get the clipboard menu item in the contextual menu in Gmail's compose email field? How can one get the clipboard menu item in the contextual menu in Gmail's compose email field?

It's not present on my phone (Samsung Galaxy S9, Android 9):



Content of the ... menu:
The clipboard is a feature of the keyboard i.e. the Input Method in android terminology. 


Assuming you are using gboard which is google's keyboard; you should see the clipboard button towards the top right of the keyboard. Depending on your device, it might also be inside the 3-dot menu item that can be clicked on to expand the rest of the list.
Clicking on this reveals more buttons. One of which is the clipboard. You could use copied items for one hour from this clipboard. In order to save items permanently, you can pin them.