However, at least speaking as an academic, I wouldn't say that ~20% acceptance rate is necessarily indicative of a shortage of good venues.
There is plenty of not-so-good research that is submitted to top places that has no hope of getting in.
(My experience is from computer security research, where the acceptance rate of security conferences has gone down, but the fraction of good papers has also actually gone down, so the fraction of good papers getting in has roughly stayed the same.)
That being said, ATC seems to indeed have been a high-prestige conference back in the day, and hence indeed competitive.
My experience is from recent years, where it was viewed as a good-but-not-tier-1 conference in systems research.
However, at least speaking as an academic, I wouldn't say that ~20% acceptance rate is necessarily indicative of a shortage of good venues. There is plenty of not-so-good research that is submitted to top places that has no hope of getting in. (My experience is from computer security research, where the acceptance rate of security conferences has gone down, but the fraction of good papers has also actually gone down, so the fraction of good papers getting in has roughly stayed the same.)
That being said, ATC seems to indeed have been a high-prestige conference back in the day, and hence indeed competitive. My experience is from recent years, where it was viewed as a good-but-not-tier-1 conference in systems research.