In the documentation https://support.office.com/en-us/article/connect-a-classic-experience-sharepoint-team-site-to-a-new-office-365-group-469c6ee0-2139-4496-9914-7e39d07ac49d

you talk about setting the policy, also the documentation on users going through the process of joining an existing SP site to an O365 Group.

In both sets of documentation there is nothing that discusses whether or not that newly enlisted site is now part of the Groups Expiration Policy.

Obviously for a Joined SP site collection we do not want this policy, or perhaps some do, but its optional.

In any case where is the documentation on what actually happens here ? We cant tell, and dont want to wait a year to find out :)