Does Manual:Searchindex table used in MW 1.27 when elasticsearch enabled？Can I safely truncate searchindex table in database?
Topic on Project:Support desk/Flow
The extension code does not contain any mention of the searchindex table. So I would guess that a. it does not modify how this table is used and b. it does not prevent MediaWiki from storing/updating records inside that table when a page is changed.
But you can check that easily: Truncate the table and then edit and save a page. If a record gets added to the searchindex table, then this table obviously is still in use. Then use the rebuildall.php maintenance script to populate the table again.
I'm pretty sure Elasticsearch uses it's own database engine for performing searches.