From Sheshant Kashyap, a New Way to Do SOQL ‘IN’ Queries in Flow
Sheshant blogs via Salesforce Binge, and has posted an installable action that lets you build up IN queries using Assignment elements:

It’s a nice addition to SOQL support extensions, which currently include:
- A Graphical SOQL Query Builder for Flow, plus an Execute SOQL Action
- Execute SOQL V1.2: Handles Flow Dates Properly and works with Flow Schedule and Record Change Triggers
- From ForcePanda: The first Invocable Action that can perform SOQL on any kind of input
- https://unofficialsf.com/easy-in-filters-in-flow-with-the-filter-query-flow-action/
- ..and the venerable Salesforce Lightning Flow — Using Advanced SOQL, which still somehow draws 50 page views a week from the old Medium blog that preceded UnofficialSF.com
Developer Note: Note the way that Sheshant leverages Apex-Defined Types to create a compact, targeted data structure that can be used in Assignment elements. I’ll state that I think that Apex-Defined Types are the biggest source of untapped opportunity to extend Flow!