Certainly not. Heads are after all ultimately responsible for what happens in school and performance management in no way changes the right they have always had to inform themselves of what is happening in classrooms, or anywhere else.
This is not to say that heads should not be courteous and sensitive in exercising this right. Normally, a teacher might expect to be warned in advance of a visit and its purpose, whih may, of course, be related to pupils and not to the teacher. Given the formality of the performance management process, it should be quite clear when visits for that purpose are taking place.
If a teacher knows why a lesson is being observed and receives immediate follow-up after it has taken place, much of the natural anxiety that is generated can be allayed.
Where lesson observation is part of the normal practice of the school and professional standards are seen to apply, one of the oldest barriers to professional development and mutual co-operation will be broken down.