So, in effect, we have a de facto ban on new third-rail, but no reported evidence to support it?
You could do an FOI request to the ORR for the evidence on which the third rail restrictions are based, if you actually want to know.
IMHO, the ORR position is unsustainable - either third-rail is unsafe (and all existing installations should be switched off, and ripped-out) or it isn't...
People keep making this argument, but it is simply not in line with reality. It is also a weird position to take that something is either safe or unsafe, given that there is a scale of things that have different levels of safety and risk. This is not a binary position, but a continuum.
It is entirely normal for a regulator to have stricter rules for new installations than for existing ones. This happens all the time, across sectors. New cars have to meet modern safety standards, but existing cars, which do not meet the modern standards, can continue to be used. The same applies in home electrical installations, certain asbestos regulations (!), and many other things.
Also, there is no de-facto complete ban on 3rd and 4th rail installations. TfL has made the case to the ORR that it was acceptable for a few of their extension.
For mainline rail, to use 3rd rail, you'd have to take steps to reduce the risks*, and give a good enough answer to the question: "
Why can't this service use OHLE or BEMUs to gain the benefits of electrification instead?"
*e.g. working practices that reduce risk for staff, removal of level crossings and other points of access, improved fencing, etc.