As far as I am aware till now nationwide lockdown is unprecedented and not supported by scientific evidence. There have until now been various enforcements of isolation on much lower scales which have been used to control disease.
Yes, a point worth stressing. During previous pandemics the most that's happened is some sort of lockdown for a particular city. Also, voices in WHO were warning against lockdowns when initially started by China. See https://en.wikipedia.org/wiki/COVID-19_pandemic_lockdowns
China, where the pandemic originated, was the first country to enforce the quarantine and lockdown of cities and later whole provinces, starting at the end of January. Although such measures are a very old tool of epidemic control,[179][180] their use at the scale of a large city such as Wuhan or the even larger scale of provinces was controversial among experts at the time, with questions on their effectiveness[181][182][183] and their ethics.[184][180][185] Some public health experts, while not always condemning the measure, raised the issue of the inevitable psychological toll such measures would have.[186][187][188] An ex-World Health Organization (WHO) official who headed the organization's Western Pacific Region during the SARS outbreak said that "the containment of a city [hadn’t] been done in the history of international public health policy".[182] The WHO called the decision to quarantine Wuhan "new to science".
As for Brooks' point, I wouldn't read too much into the fact that many other countries followed this trend of lockdowns. Once major European countries were doing this, it would look careless not to impose something. Just look at the flak the UK government got before they imposed their lockdown.
This type of social experiment ... is still on an unprecedented level. I can't recall any time before now we have been banned from associating with others except in military occupations. Surely that is a concern?
Absolutely, on many levels. It's economically very destructive (I'm talking of the real economy, not the FTSE company shares losing value), the culture of fear -- especially for children, the lack of an end (there may be a second wave in the Summer/Autumn/Winter, then a third wave), and as you've already pointed out, the toll on mental health through the lack of human interaction. Also the very basic freedom of taking risks we are now in danger of losing. We could get to a world where risks will be assessed by officials/managers who could impose measures based on their own fears. (Of course, to some extent this happens, but the scope has increased incredibly). What's happening in universities is a good example of this. All buildings are locked and staff are forced to work from home, even though there are no students around and the places are largely empty.