I would like to recommend a different approach: Suppose you have or make a top-level parent label, for example "PROJECTS BY YEAR", and within that you nest your "Year 2014", "Year 2015", "Year 2020", "Year 2021", etc. Create a new label called "ARCHIVED PROJECTS" that exists at peer-level to your various YEAR folders.
Then when you are working within, say, "YEAR 2021" and its various sub-level PROJECT FOLDERS (P1, P2, P3, etc), when you complete or close-out P1, you then reassign its parent label to be "ARCHIVED PROJECTS".
It's not as elegant as OP's desire to "Hide" completed projects, but it seems like it might serve OP's original intent to reduce the number of "done" or "inactive folders" from clogging up your active folders list. Yes you would be adding a Label/Folder called Archived Projects, but if I understand your objective (and I may not!), this would clear out finished Projects from appearing in your Current Year Folder.