Atlassian, please consider implementing this feature.  Perhaps it could be added as a template option in the 'Smart Button' Element.

I need this feature for checklists (info in Wikipedia)

Any chance this will be added soon?

I have tried the two Javascript/jQuery below in the Chrome console, and while the former does indeed remove every checkmark, it does not save them, so after refreshing the page they are again not un-checked (FYI: only tested in non-edit-mode, since that's where I need it).

I can not add custom macros or HTML, so it needs to be a built-in feature.

I would guess the best way to implement this feature is like the "expand" macro, where a check/un-check all would do that action to every task inside the macro?

Or maybe a tag on every task could check/un-check all tasks wherever they are on the page? That way, you don't have to worry about if the task is inside tables or other macros.

+ 1 for this!

I hope this gets release by 10th anniversary of this issue. If so, we have 1.5 years to wait, not so much

This is definitely needed, especially controlling the indent task boxes by the main task box. 

My team and I would like to upvote this!

Amazing Ben! Works great in Firefox. Thank you for sharing!

The answer above using jquery didn't work for me, but this does:

Array.from(document.querySelectorAll('.inline-task-list li')).forEach((el) => el.classList.remove('checked')); 

Should work on all modern browsers.

This didn't work for me; checkboxes in tables and outside tables were unaffected. 

Open developer tools in your browser and go to the console and paste this code in and hit enter:

$('input[type=checkbox]').prop( "checked", false )

Works for me as of Feb 2022.