ASP.Net Core maxUrlLength

You are correct that maxUrlLength configuration value not available is not available ASP.net core the way it is available in previous version. The clean reason for this is that previous version only support IIS and Windows so it is tightly integrated with that. Now It is supported with other OS and to do that the way they did it reverse proxy with actual server. Like In Windows IIS and Linux NGinx.

1. IIS will communicate with Kestrel.
2. NGIx will comminicate with Kestrel.

Now any request filtering or url setting we have to do is at IIS or NGinx level.

If you are working in Windows you will find "Request Filtering" feature and for that you have to add Web.config file. ( I have not tested NGInx)

You have to do something like this.

<system.webServer>
    <!--   Here you have other settings related to handlers.-->
   <security>
      <requestFiltering>
         <requestLimits maxAllowedContentLength="3000000" maxUrl="10241" maxQueryString="20481" />
      </requestFiltering>
   </security>
 </system.webServer>

Maybe this can help

KestrelServerLimits.MaxRequestLineSize Property

https://docs.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.server.kestrel.core.kestrelserverlimits.maxrequestlinesize?view=aspnetcore-3.1#Microsoft_AspNetCore_Server_Kestrel_Core_KestrelServerLimits_MaxRequestLineSize