Sharepoint - SPFx Version support for On Premise Server (2019)

SPFx version 1.4.1 is supported with 2019. This will likely not be updated to any newer version of SPFx.

Feature-wise, I don't believe this is documented in a single location. You would need to review each feature release, and even then there are going to be cloud-only features that aren't going to be available in SharePoint 2019.

Supported SharePoint Framework version for the SharePoint 2019 is SharePoint Framework v1.4.1.

Read this article for details around supported development capabilities in the SharePoint 2019:SharePoint Server 2019 development platform

Besides, you could refer to here for each version of SPFx.