SharePoint 2019 offers similar app model and server infrastructure to SharePoint 2016-2013 in terms of web part registration and usage. This means that you can install manually and use the available SharePoint 2013 Telerik web parts under SharePoint 2019.
You should be able to manually install the Telerik AJAX Web Parts
via the provided Telerik_Web_Parts_For_Sharepoint2013_2019_3_917_Dev.zip -> RadControlsSP.wsp setup file.
Here is what we found regarding the ASP.NET Web Forms level of support for SP2019:
As per our observation, the old structure is still present in SharePoint 2019 and therefore, we can continue using the Telerik SharePoint Web Parts same way. The TeamSite type of site collection has a Modern layout, but SharePoint 2019 kept the classic one too. We have tested all major scenarios, using the built-in web parts, binding to data (SQL, SharePoint lists), creating custom web parts, enabling AJAX.
Here are some screenshots:
Insert RadEditor Web Part