When right clicking on a places folder in Dolphin, the context menu shows “Remove” to remove the folder.
I personally think this is a bit confusing as you might think it deletes the folder instead of removing it from places.
I think elaborating the name to “Remove from Places” would be ideal, especially since the context menu option for adding a folder to places when you right click on one is called “Add to Places”.
Also the symbol could be changed to the bookmark icon with a minus sign instead of the trash symbol because the add option is like that with a plus.
Finally, the color for the symbol could be black instead of red because the action can easily be reversed. The images below show what it currently looks like for both.
Side note: It was also difficult to capture a screenshot of the context menu because when using spectacle to capture the active window it would disappear in the screenshot. I had to capture the whole screen for it to show for some reason. I guess this is because the meta key in the shortcut must close the menu.
The thing with the term “pinning” is that it might imply that folders are temporarily placed in places and pinning them makes it permanent but places doesn’t really function that way to my knowledge - they are either there or not there until added/removed by user input. It is different from the taskbar in that regard.
(I’m thinking about the Windows 10 analog called quick access which uses “pinning” - folders are also placed there if they are visited often without user input.)
Personally, although I understand that this could be a misuse of the term “Pin” (because folders don’t ever temporarily add themselves to Places automatically), I think the advantages of using Pin outweigh that disadvantage. Namely (as has been said) the fact that it is consistent with pinning things to Taskbar, and for similarity to Windows Quick Access terminology.
Note that, in Windows, the right-click menu entry on any folder is “Pin to Quick Access” whether or not the folder is already in Quick Access due to frequent access. Similarly in KDE, you can pin an application to the Taskbar, even if you’ve never run it before. So maybe the term “Pin” can be used more widely with no confusion.
TL;DR I vote for “Pin to Places” and “Unpin from Places” terminology.