Nope, could be made with <strike> as well, but swapping element tags takes more characters than toggling a class. In the case of UmbrellaJS I made a .wrap() method, but since there is no .unwrap() again it takes more than toggling classes.

Ah, fair. The challenge itself didn't mention toggling (just crossing tasks off), so you could probably get away with it; But that's just the golfer in me talking I guess.

I also thought of using the <s> tag, but I didn’t use it because it is considered to be an outdated tag

But yes that is totally possible