Previously, when searching for a block type in the inserter, the number of search results (or "no results") was made available to users of assistive technologies by sending a message to the assertive live region. The screenshot below is from Gutenberg 2.5:

Since then, the inserter has gone through various iterations and is now notably different. However, the several changes to the UI have apparently missed to re-implement the audible messages. On current master 4.0.0-rc.1 the search results are not announced at all, no message is sent to the live region.

This is a pretty important regression, as now users don't have a clue what happens when they type something in the search field.