Here's one way they do it that isn't documented or widely known: an account can get "searchbanned". While your account is searchbanned, your new tweets can't show up in other people's search results unless they follow you. There is no indication when you've been seachbanned or when the ban is lifted, and no documentation on its existence or how to get un-searchbanned. We know because we gather community input on a specific hashtag, and have gotten complaints that specific people's contributions weren't included, because they didn't show up in the search.

Interestingly, we had one person's account whose search results still showed searchbanned tweets. They would not show for that person if they logged out. We also could not find out why that person's account in particular could see them.