Oh, you actually mean “folder,” as in the Folder view (so many people say ‘folder’ when they mean ‘library,’ so my mistake).
When in Folder view, when you hover over a track, the ‘select’ tool appears to the left:
There has never been a ‘Select All’ button. As the How-To explains, once you select one item, you are in ‘Select’ mode. You can then select additional items one by one, or hold the Shift key and select another item, and everything between them will be selected (this is just like the Windows File Manager).
So to select every track in a folder, click the Select tool on the first track, then Shift-click the last track. Every track between them will be selected, and you can add them to a playlist.
This method isn’t ideal in a folder with a large number of tracks, unfortunately. There is a feature request to add a ‘Select All’ button. You may want to add your vote.
