Is shared update required to edit feature services shared in an ArcGIS Online group?

No. A common misunderstanding is that groups must enable shared update for members to edit feature services through apps such as Collector for ArcGIS. Although members of a shared update group are allowed to edit the shared items, it is not a requirement. Editing is allowed by sharing the feature services according to user roles, or through item settings with shared update disabled. Refer to ArcGIS Online: Manage hosted feature layers for more information.

Note:
If edits are made through apps such as Collector for ArcGIS, Esri recommends enabling editing from the user role and through item settings rather than through a shared update group. This is because most ArcGIS apps do not support updating items in a shared update group. Refer to the app documentation to determine if it supports the shared update capability.