I did a quick bit of Googling a while back as I was wondering the exact same thing. I seem to recall finding that it used to be a requirement a while back, but isn't anymore. I'm sure someone more informed may correct me or add some more info!

I do wonder if there is any point to a poster - if some gets electrocuted, do you really have the time to read a poster on the wall before taking action?

The Electricity (Factories Act) Special Regulations 1908 & 1944 required an Occupier of a factory to display an abstract of the regulations and the electric shock placard.  These requirements were not included in the Electricity at Work Regulations 1989.

However, the guidance on the regs says:

Emergency resuscitation and first aid

224 You should consider placing notices or placards giving details of emergency resuscitation procedures in the event of electric shock at those locations where people may be at greater risk of electric shock.

Guidance to Inspectors states

(3) There is no specific requirement under the EAW Regulations for the display of an electric shock placard or an abstract of the 1908/1944 Regulations. Occupiers should be told to remove abstracts but advised to retain placards where these are appropriate

I am impressed with Granlund40055’s research! Just thinking about it could it be that back then (probably 1908 and maybe even in 1944) people less familiar with electrical hazards and  for example might not realise that for example if someone was being electrocuted what you don’t do is grab hold of them.

The suggestion is that such a sign might still be necessary is in the current guidance and I suppose whether you need one should be down to a risk assessment. If that is the case why did the original posters audits insist that he needed one?