I don't know how many people are using Bootstrap anymore. Professionally I know that the only time we use Bootstrap, in a medium sized corporation, is if it's already bundled as a dependency for some other package we're using. I would imagine that the 20% of the internet still using Bootstrap is probably going to drop significantly with the removal of IE support. Or maybe not, maybe they're aware of the impact not supporting IE will have and found it to be a reasonable path to go down.
Things like Bulma and Material offer less features, though bring some conveniences.
You don’t have to update bootstrap if you want to keep supporting IE, I don’t see anything wrong with that, you might just miss some conveniences, so I don’t think it will affect them that much, need IE? use bootstrap 4
This isn't as terrible as it seems since many of those sites won't update their version of Bootstrap.