Shadowbanning is a quiet way of diminishing a person's presence on a platform, making it so that only that person can see the content they post, not their followers or searchers. But that's not what's happening here: Twitter's search isn't autopopulating some handles, even when they are typed into search directly. BuzzFeed News found tweets from prominent left-leaning accounts were equally subject to this kind of treatment.
Twitter has blamed the issue on a "bug," which is a result of its endeavor to encourage civil and healthy conversation on the platform. It's not clear exactly how these accounts were flagged, but Twitter's head of product, Kayvon Beykpour, confirmed via Twitter that there was no partisan mindset at work here. "Our behavioral ranking doesn't make judgements based on political views or the substance of tweets," he said.
The bug in question is why users couldn't search for the accounts. "Our usage of the behavior signals within search was causing this to happen & making search results seem inaccurate," Beykpour continued on Twitter. A fix should currently be in place, so you should be able to search for these accounts as much as you want now.