Tampermonkey script to check and replace youtube's subscriptions to show all videos
A short review;

BadDumbYoutubeURL should be lowerCamelCase and perhaps is a bit silly

the ; does not deserve it's on own line

My understanding is that location.search already starts with ? so you probably want to do something about that, like
 const search = window.location.search ? '&amp;' + window.location.search.substring(1) : 0;


Other than looks fine, perfectly maintainable