we need to be asking if this degree of privacy serves the British people well? Especially when there are archaic traditions such as Erskine May built in to our systems
What Erskine May (not Mayne) actually says
'As indicated at para 20.10, unless the discussion is based upon a substantive motion, drawn in proper terms, reflections must not be cast in debate upon the conduct of the Sovereign, the heir to the throne, or other members of the royal family.1 The same principle applies to the Lord Chancellor,2 the Governor-General of an independent territory,3 and judges of the superior courts of the United Kingdom4 (including persons holding the position of a judge, such as circuit judges and their deputies, as well as recorders).'
The following paragraph explains whose conduct is limited by the rule.
You will see that the limitation is simply that if you want to criticise these people you should do it properly, in a substantive motion, which can be fully debated. I don't see any difficulty with that. It stops parliamentary time being wasted with unsubstantiated rumours and gossip, and allows MPs to table motions for debate if they wish.
If MPs don't wish to table motions, that's another matter, but there's nothing in Erskine May to stop them.