Removing a modification done under an STC

You remove the item and make a mechanics logbook entry, notifying nobody. A mechanic does not need approval to move an aircraft from one certified configuration to another for which there are no added rights to be purchased.

For a simple “install this electronic box” type of STC that probably works. Simply take it out and make a logbook entry. But for a more complex STC I fear a removal must be engineered. In a best case the original STC designer already integrated a removal procedure and gets that approved with the STC. If you drill a hole for an antenna and put in a doubler plate there is no going back. If the antenna is removed some blanking plate will be required. Most mechanics will simply cut a piece of metal and screw it down but the more complex the modification gets the more difficult this might be.

If the aircraft is returned to its originally certified configuration, it is certified.

Modifications to the certified state like doublers over antenna holes are typically done based on FAA AC 41.13, which like STCs is approved data, and obviously have zero to do with the STC that has been removed or its owner.

Here is the relevant volume of AC41.13.

Page 4-32 would be a good place to start if you had to do any sheet metal work after permanently removing an STC item.

It is FAA approved data.

We always had an aircraft engineer with us whose job it was to rig and derig cameras and their mounts to an aircraft. AFAIK most did not require an STC in order to do it. I believe there was a special authorization required which contained the details of the fixing to the aircraft, weights, balances etc which had to be approved as each mount eg pod ,camera, and fixing were different.
The only times I can think of, when an STC might have been required, and in which case, already stated the attaching and demounting would have been covered by an STC was the case of commercials such as the ones directed by Tony Kay for British Airways when a hole had to be cut in the floor of a 747? to mount a pod and camera below so that they could film other BA aircraft above the Alps and other places of interest. The problem which may or may not have required an STC was one of placing what was essentially a periscope through a pressurised cabin area.
The only other time, I can think of where an STC might be required would be on police helicopters with their camera pods being a more or less permanent fixture. But I believe these are part of the TC as they are ordered for the particular job. But I may be wrong in this as I’ve always had engineers whose speciality was fixing cameras to aircraft.

Many years ago I was involved in operating this thing below on a C172. I remember it had an STC but could be attached and removed by the pilot. Also train your left foot if the plane has no rudder trim.

If the antenna is removed some blanking plate will be required. Most mechanics will simply cut a piece of metal and screw it down but the more complex the modification gets the more difficult this might be.

Yes; the Q hangs on whether creating a blanking plate is a Minor or Major mod. In the FAA system, unpressurised, it is Minor and is done under the AC-41.13 standard procedure manual. Done by an A&P and a logbook entry. If EASA has a similar manual then you can use that. Otherwise, it is Major

What complexity the original mod was is not relevant. It is the complexity of the restoration work that matters.

Might help if the actual mod being removed was described.