Insider level Version Current Build Release Date
Insider
(formerly Insider Fast)
1804 9226.2021 March 29, 2018

Version Release date Highlights
Version 1804 (Build 9226.2021) March 29, 2018 Various performance and stability fixes

Source: Update history for Office Insider for Windows desktop - Office Support

See also: Insider release notes for Outlook 2016 for Windows desktop - Office Support