The Black Death or Detroit. I don't know which is worse.
I keed, I keed: many very smart people are doing really interesting things in Detroit. It's interesting to note, though, the difference between extrinsic and intrinsic causal factors: the Black Death significantly increased real peasant wages and per capita wealth, likely helping the transition from a feudal to capitalist society through dispersing capital more widely. It's a much better situation (for the people who remain) than the Detroit case, where it was low returns to labor that depopulated it.
Estonia is much closer to the Detroit case, it sounds like, which is not necessarily a good thing for its future prospects.
I'm certain there are other historical mass migrations of such enormous magnitude, but my point was really to glibly provide perspective for such a massive population decrease.
I would assume many/most of those emigrated people are in Scandinavia. It is a bit smaller step than moving to USA in the 19th century... (And much easier to move back, when the economy allows it.)
A more relevant parallel might be many internal moves in big countries to industrial areas, when industrialisation started.
I don't think it's completely unprecedented, but I doubt that people will take heart in being compared favorably to the Black Death.
https://en.wikipedia.org/wiki/Black_Death