This is from a news article.
Outside music, Sakamoto was known for activism — and in particular for his anti-nuclear views, which saw him demonstrating against nuclear power plants and co-organizing a "No Nukes" concert in 2012.
I think the antecedent of the relative pronoun which is the entire previous clause.
If so, I wonder what the meaning of the saw is in the above context.
I can't find any definition of 'see' in the dictionary which is appropriate in the above context.
I think you are distracted by the odd punctuation of the parenthetical clause 'and in particular for his anti-nuclear views', which (in my opinion) should have been preceded and followed by parentheses, em dashes or commas, instead of an em dash before and a comma after. If we remove that clause — as we can with parentheticals — we see that 'which' refers to Sakamoto's activism, and the meaning is as here:
(transitive) to be characterized by
this period of history has seen much unrest
Of things, places, etc.: To be contemporary with and in the neighbourhood of, to be the scene of (an event); to be in existence during (a period of time). Also of a period of time: To be marked by (an event).
In 1906 Cambridge saw three or four of her most learned men compete for the Greek chair.
Oxford English Dictionary
