First of all, this is a good initiative TryNinja. It looks simple but it's a powerful improvement to the forum experience. Being able to fully ignore users, including removing their quoted content, is a feature many members of the forum have wanted for years.
I think I might be the first to install this on my device, though I don’t have anyone to ignore at the moment.

I have one observations though; Why did you decide to replace the quote instead of completing removing the quote in the script below?
Instead of the Original
if (list.includes(authorMatch[1]) && quoteBody) {
quoteBody.innerHTML = 'USER IS IGNORED';
I tried this as it removes the quote block completely
if (ignoreList.includes(authorMatch[1]) && quoteBody) {
quoteHeader.parentElement.remove();
I suppose the outcome would be the same, I happened to make an observation.