I think the obvious thing to do is to simply invite any fellow passengers to come to FT to find out more. [And other websites exist, I guess].

And I would tell someone who looked concerned in this situation, without making a bad situation worse by disrupting things, but this sort of thing applies to many things in life. Very few people claim the married/CP transfer personal allowance, for example, but it is mentioned on the self assessment forms, websites, financial websites, Moneybox and so on. How many people do their EH111 or even know what it is?

It's not clear from the wording but if it was LHR T5B, there is a sign about EC261 on every gate, I checked fairly recently. But as with many signs it's not often spotted. BA introduced new and fairly clear mention of EC261 during the OLCI process a few months ago too, and again I don't recall anyone mentioning the change here. I guess you would prefer it all to be automated, which in terms of public policy is probably best, accepting that the cost of travel would slightly increase as a result of that. The down side is that with many airlines operating very close to the wire, and many travellers close to the wire in terms of what they can afford to spend on travel, that too has potential downsides.