His most played works are (I think) symphonies 5 and 7. The 7th was written in 1941. That said, I've seen a bunch of his later works performed too.
But this is just nitpicking. Without a good definition of "standard orchestral repertoire" and a good dataset of orchestral performances we're just making things up.
It was a claim I heard numerous times as a music student, and find it to be a pretty good fence-post. I always understood it to refer to the "usual suspects" that turn up on concert programs, but it would be interesting to work out what today's standard orchestral repertoire actually is by collating and analyzing orchestral concert programs.
I'd say Shostakovitch 5 might be considered to be part of the "standard" repertoire, but that was composed in the '30s. Maybe his later symphonies turn up more frequently on programs today than does Bartok's Concerto for Orchestra - I'm skeptical but don't actually know.
But this is just nitpicking. Without a good definition of "standard orchestral repertoire" and a good dataset of orchestral performances we're just making things up.