When comparing with SharePoint Server 2019 with SharePoint Online, more than 50% of elements and components may differ. Lot of items are not available in SharePoint 2019 including modern OOTB SPFx webparts.

I have prepared the compilation of available / unavailable webparts in both environments based on the analysis.

MODERN WEBPARTS

WebParts SPOnline SP2019
Bing Maps Y N
Button Y N
Call to Action Y N
Code Snippet Y N
Countdown Timer Y N
Divider Y Y
Document Library Y Y
Embed Y Y
Events Y Y
File Viewer Y Y
Group Calednar Y N
Hero Y Y
Highlighted Content Y Y
Image Y Y
Image Gallery y Y
Kindle Instane Preview Y N
Link Y N
List Y Y
List Properties Y N
Markdown Y N
Microsoft Forms Y N
News Y Y
Microsoft Connectors Y N
Microsoft 365  Video Y N
Page Proeprtoies Y N
People Y Y
Planner Y N
PowerApps Y N
Power Bi Report Y N
Quick Chart y Y
Quick Links Y Y
Recent Documents Y N
Site Activity Y Y
Sites Y N
Spacer Y N
Stream Y Y
Text Y Y
Twitter Y N
Weather Y N
World Clock Y N
Yammer Y Y
Youtube Y N

If I missed any webparts, do let me know in the below comments section. Will cover the alternative webparts / solutions in my next blog post for each Unavailable webparts.

Reference: