Could this be to do with standards, i.e. an extension of the recent-ish de-listing (and subsequent reinstatement) of RCD sockets from BS7671 as a suitable method for additional protection?

I can easily envisage someone getting the wrong end of the stick and interpreting that as "they're not allowed any more full stop"

Or building on mapj1's suggestion, perhaps there're legitimate concerns that many RCD sockets* are in fact still type AC and it's easier for the author to still not allow it as a solution than to control the spec. And with RCD extension leads, depending on the application, it's quite forseeable that users will helpfully swap a faulty one for another from home or the local shops, which may or may not have equivalent protection, so again easier to blanket ban as a solution in a risk assessment.

*In all honesty I have no idea of the type RCDs fitted into extension leads would be.

Could this be to do with standards, i.e. an extension of the recent-ish de-listing (and subsequent reinstatement) of RCD sockets from BS7671 as a suitable method for additional protection?

I can easily envisage someone getting the wrong end of the stick and interpreting that as "they're not allowed any more full stop"

Or building on mapj1's suggestion, perhaps there're legitimate concerns that many RCD sockets* are in fact still type AC and it's easier for the author to still not allow it as a solution than to control the spec. And with RCD extension leads, depending on the application, it's quite forseeable that users will helpfully swap a faulty one for another from home or the local shops, which may or may not have equivalent protection, so again easier to blanket ban as a solution in a risk assessment.

*In all honesty I have no idea of the type RCDs fitted into extension leads would be.