I wish to apply a keyword to multiple documents. Selecting those documents and dragging a keyword into the keyword pane should, I believe, do that. In my version ( 220.127.116.11 Beta (861203) 64-bit - 11 Mar 2020) this appears to apply the keyword only to the first document.
I tried to search here for info on whether this is a known, yet-to-be-implemented feature but was unsuccessful.
With multiple documents selected in the binder, right click on the keyword(s) you want to apply and select ‘Apply Keywords to Selected Documents’. You can use this context menu to remove keywords from multiple documents as well.
BTW, if you click on a keyword then on the magnifying glass you can search all documents for that keyword (or for the underlying word itself, or for documents with no keyword) and it will collate all the matches in a temporary collection in the binder.