Our goal is to enable Extranet connectivity for SharePoint 2019 with support for the OneDrive Synchronization client (also known as the Next Generation Synch Client - NGSC).

During our investigation we found that, while the NGSC worked successfully with NTLM authentication worked, it did not work with SAML claims authentication.

SharePoint 2019 supports the use of SAML 1.1 but the NGSC does not. The product group is investigating the possibility of adding that feature but currently there is no ETA.
During our call with MS Support we verified that the NGSC does not even pass cookies;
However when we are using persistent cookies, we successfully mapped a network drive to the OneDrive share but this is not the desired user experience.