As of January 1, 2020, Twitter's Terms of Service state that they:
(...) may also remove or refuse to distribute any Content on the Services, limit distribution or visibility of any Content on the service (...)
QFD caused your tweets to be invisible within the latest section of the search, including hashtags, when the quality filter on the search page was turned on. The filter was turned on by default and would reset for each search anew. QFD was introduced on May 15, 2018 as part of Twitter's so-called healthy conversation project.
The quality filter was deprecated shortly before Twitter CEO Jack Dorsey testified before the United States House Committee on Energy and Commerce. In his testimony, Dorsey stated the following:
Twitter recently made a change to how one of our behavior based algorithms works in search results. When people used search, our algorithms were filtering out those that had a higher likelihood of being abusive from the “Latest” tab by default. Those search results were visible in “Latest” if someone turned off the quality filter in search, and they were also in Top search and elsewhere throughout the product. Twitter decided that a higher level of precision is needed when filtering to ensure these accounts are included in “Latest” by default. Twitter therefore turned off the algorithm. As always, we will continue to refine our approach and will be transparent about why we make the decisions that we do.
The quality filter has been removed entirely, but please note that this is not a verification that users are not classified anymore by this algorithm. We just cannot observe it anymore through the quality filter.
For documentation purposes, you still find the frequently answered questions about QFD here:
The Twitter search features several search modifiers. One of them is the prefix from: which allows to search for tweets from a specific user. For example, when checking whether @shadowban_eu has a shadowban, we query the Twitter search for from:@shadowban_eu . If we do not find any tweets although the user has tweeted in the past, the account is subject to a search ban.
When the user has a search ban, the tester searches their profile for the latest tweet with at least one reply. It then visits the direct URL of the tweet and selects some reply tweet. Afterwards, the direct URL of the reply tweet is visited. If the tweet that the user replied to is not visible, the likelihood that the account is affected by a thread shadowban is very high.
When you upload an image, cite a tweet or link to an external page, Twitter generates a shortlink for that content.
We first find a reference tweet using a combination of from: and filter: search modifiers. For example, from:@shadowban_eu filter:links returns all tweets from @shadowban_eu containing a link.
We simply pick one result tweet as reference and extract the shortlink, e.g. https://t.co/5Vzu63ypQJ .
(This link points to a tweet, quoted here).
We then query the Twitter search for the tweet using the shortlink's URL with the quality filter turned on and off. If the reference tweet is found in both cases, the account is not affected by QFD. In case it is found only when the filter is turned off, it is affected.
We chose this method because shortlinks are unique. Methodically, it does not differ from searching for text or a hashtag but the uniqueness of shortlinks is a technical advantage.