Why model binding to JObject from a request doesn't work anymore in ASP.NET Core 3.1 and what's the alternative ?

Json.Net (NewtonSoft) has been for a long time the most used JSON serializer in .NET world. Since .NET Core 3 and ASP.NET Core 3 Microsoft introduced a new one named System.Text.Json. JObject is a class that belongs to Json.Net (NewtonSoft) and if this latest is replaced by System.Text.Json, in this case you should expect that using JObject will no longer work.