I think you worry too much. Has there ever been a case of a website being held more responsible for advice than "some bloke I met in the pub told me that..."?
I don't know - probably not - but I wasn't really thinking primarily about legal liability (so probably should have picked a better word than 'disclaimer').
Mind you, I would have said that what we're talking about is rather different from advice being given in real time, which is subject to the 'bloke in the pub' argument. In the situation we are discussing, DIYnot is, without any specific warning (and without 'needing to'), "going out of its way" to bring to people's attention a thread which it knows (or should know) could contain information which is no longer correct. If a pub landlord ("deliberately and unnecessarily") brought to the attention of the public that there was "an electrician in the corner of his bar giving advice", in the knowledge that the person was sitting there clutching a copy of the 15th Edition (1981) of the Wiring Regs, I don't think that landlord would be beyond criticism, morally if not legally!
Anyway, as I said, I wasn't really thinking of legal liability but, rather, of trying to be helpful to members. DIYnot has instituted this 'big red warning' pointing out that the old thread "may not need any further discussion", but without pointing out it may contain information which is no longer correct. Of course, that message is only seen by those who attempt to
reply to an old thread - ideally, they would be 'warned' when they merely
read a very old thread.
Kind Regards, John