Use ISubScriber in TFS2012/2017 works fine, but cannot find namespace and dll for WorkItemChangedEvent Type in TFS 2019 (DevOps Server 2019

Up to TFS 2017 there was a library in Plugins folder called Microsoft.TeamFoundation.Framework.Server it contained ISubscriber interface which one could override in order to attach hooks and add behavior for events such as build completed or new push to the git repository. The library is missing in TFS 2019 (DevOps Server 2019). Does anyone know how do we maintain our plugins now?

using Microsoft.TeamFoundation.Framework.Server;
using Microsoft.TeamFoundation.Common;
using Microsoft.TeamFoundation.WorkItemTracking.Server; 

        public Type[] SubscribedTypes()
        {
            return new Type[1] { typeof(WorkItemChangedEvent) };
        }

Cannot find matched dll and namespace for WorkItemChangedEvent type.