GSuite Legacy Free Edition no longer supports email to Groups from outside domain

One of my clients is still using the legacy free version of GSuite legacy free edition

Prior to December 6, 2012, Google offered a free edition of G Suite-also known as the legacy free edition-that had a reduced set of business features. As of December 6, 2012, Google stopped offering the free edition to new customers.

I followed the steps in the Google FAQ to configure a group “info@” email address (using Google Groups)
Unfortunately, I couldn’t get email posting from outside the domain to work. Every time I sent an email to info@ from an ‘outside’ email address, it bounced back:

After looking through all the settings, I saw Allow users to post to the group on the web was greyed out

It appears that this is no longer an option for legacy ‘free’ G Suite domains — as indicated by

Web posting is currently enabled only on G Suite domains.