That seems high?
I expect "80%" possible and even plausible, but the one piece of data which I'm going to present (below) suggests "40%",
There's data referenced in Uncounted Costs of World War II:
The Effect of Changing Sex Ratios on Marriage
and Fertility of Russian Women, which says there ended up being a male/female sex ratio of 0.6 to 1 for that cohort.
IMO that implies that "80%" is a bit high (because 80% would imply a 0.2 to 1 ratio assuming no women died).
On page 5,
Russian demographers calculate that the probability of surviving
between 1941 and 1946 for men aged 25 to 34 fell from .96 – the probability in the absence of
the war based on 1940 mortality rates – to .61 (Andreev et. al. 1993).
While women in the prewar Soviet Union already contended with sex ratios below 1.0 – likely
due to the revolutions of 1905 and 1917, World War I (1914-1917), civil war (1918-1922) and
the political purges of the 1930s, all of which disproportionately affected men – the sex ratio fell
dramatically for individuals born around 1925, from .91 to .65 for the 20-29 age group.
But these are numbers for Russia, not the whole Soviet Union:
The data and figures in this and following sections are for the Russian republic (the RSFSR)
rather than the Soviet Union as a whole, since the empirical analysis uses Russian census data and
primarily focuses on the Russian republic.
It's easy to imagine most of the men of fighting age dying, in places like Leningrad.
Russia accounts for maybe half the population of the USSR, the next largest being the Ukraine. Given that there was mass conscription I'm going to guess that Russian losses were representative of those of the USSR as a whole (though it also wouldn't surprise me if Ukrainian casualties were higher).
Still I don't see how or from where to infer 80%. Perhaps (I speculate) the pre-war army was nearly annihilated, and that every man born in 1923 was (aged 18 in 1941 and) a conscript in that army:
The surprise attack on
the woefully unprepared Red Army led to devastating losses for the Soviet Union in the early
phase of the war: within the first six months, the Red Army had lost nearly 5 million men – the
size of the Soviet Union’s entire prewar army – and had lost territory equal to the size of the
United States between the East Coast and Springfield, Illinois (Glantz 2005).
The Soviet Union mobilized all possible resources in its subsequent fight for survival and
ultimate victory. The need for manpower dictated a significant loosening of the age and
nationality restrictions on conscription of Soviet citizens; it is reported that men “well under” the
age of 18 and exceeding 55 years of age were conscripted into the Red Army, with Russians and
non-Russians alike required to serve (Glantz 2005).
One anomaly that is not explained is there is an apparent discontinuity in the graph for Figure 3 on page 30:
But perhaps that's not proof that the numbers are 'off the scale' then, but rather it's because there are no census numbers for that date-range ("at age 20"), because it's while the war is actually happening.