Hi hspinto, for existing tenants, there is a separate toggle in the security.microsoft.com portal to switch over to streamlined for "API-based" methods. This applies to both Intune and Defender for Servers, but they won't automatically re-onboard devices.
The extension will do different things based on what OS it finds (install fresh on 2012 R2, 2016, use the inbox sensor on 2019+) so answering your question with specifics is trickier but note the version requirements mentioned above (March 2022 but I sincerely hope your servers have received more recent LCUs/KB5005292 than that). If version requirements haven't been met, you can likely still onboard but will have to make sure to patch up to get everything working correctly (note that goes for feature functionality in general!).
Finally, yes, the service tag is now GA but it's very important to understand that there are Defender-adjacent services like Windows Update, CRL, you may want to make accessible for servers as well - else ensure alternative approaches. Onboarding devices using streamlined connectivity for Microsoft Defender for Endpoint - Microsoft Defender for Endpoint | Microsoft Learn - that guidance applies.