Some groups, depending on the plan chosen and the date the group was formed, have more features and amount of storage. All groups have comprehensive moderation tools and message archives. 

Check the dates that some basic (free) group functions were changed and were legacy enabled (grandfathered).

For a full list (current as of Nov 26, 2020) see Files.

https://groups.io/g/GroupManagersForum/files/Groups.io%20features%20%28includes%20legacy%20stuff%29.pdf