Effective November 1, 2019, United Club customers, including members and their guests, and one-time pass holders will need to provide a same-day boarding pass for travel on United, Star Alliance™ or a contracted partner for entry into all United Club locations
https://www.united.com/web/en-US/con...e/default.aspx
When DL first announced this starting Jan 1, 2019, UA didn't follow, but now that AA is also requiring AA/OW/partner boarding pass, UA is completing the trifecta.