Yes I am seeing the same issue with my paid account! I shared one note with another free account (I'm trying to show them how cool Evernote is. :^( ), but when I try the second it just says pending. What gives???!
Looks like this feature was added: Select everything you want check or uncheck. Does not matter if you include text that does not have checkboxes. Do a long right click (hold it down for a second or two) on the selected area and release. A drop-down window should appear where your mouse right clicked. Move your mouse pointer to "To-Do" and then click either "Check All" or "Uncheck All". That should do it. Note I am using the Windows Desktop version of Evernote 18.104.22.16825 (305825) Public (CE Build ce1.28.2184)