We use publicly available reference data that is hosted by various government organizations in our maps to show relevant background information for our field work. Since we are not the owner of that data, we are not able to change the sync settings to allow for use offline.
The only workaround is to download the publicly available data, clip out the area that is relevant to the work being done, and upload it as a separate hosted feature layer. It is frustrating that data that is already publicly available and hosted needs to be duplicated and subsequently stored in AGOL for use in offline maps.
As these layers are not editable and for those I use, are relatively static, I do not see why they couldn't be made available offline in some capacity, even if that was only a static copy from the time the offline area was created that would need to be manually updated.