It is fairly common used on it’s own in the UI and many other places I have seen this used as the hollow with the solid indicating it is “Bookmarked”. So not really needed have an additional label. Pressing the symbol pulls up the bookmark menu.
FB & Reddit call the Bookmark symbol Save as in Save page. So yes quite commonly used.
Actions most commonly used are recognized by the symbols used. Click on the bell the dropdown gives the symbols with names. Reply doesn’t need the work reply as it is not used on the individual posts which is the same with bookmark.
If these were new unknown not common features used ie completely new.