Seems to be in the nature of things that there's a relatively limited list
of anything that's fairly recent- and last-half-century is fairly recent-
that's frequently performed.  This despite the fact that composers of the
last half century have written in a bewildering variety of styles and a
large number of pieces that are both good enough and accessible enough to
find a ready place on concert programs are not hard to find.  Or perhaps,
because of that fact.

To the list already mentioned- well, I don't know if "since the 1950s"
can include the late 1950s; probably not.  I would have included Rubbra's
concerto in G (pub.  1959) to works mentioned that, not frequently
performed, ought more often to be (since it's fair to say the topic has
diverged, as often and fairly happens, though the original topic is worth
returning to); Golubev's 3 piano concerti (#3 written for and recorded by
Tatiana Nikolaeva) I can't date properly (the fact that his first concerto
is op. 24 and his 3rd piano sonata op. 9 was published in 1959 doesn't
tell me for sure that it was .written. around then, alas, for late
published works are common in any time and place...  and I do not have a
reliable worklist anywhere; the music itself I know from piano reduction
and recommend highly, as I do all his music- quartets, symphonies,harp
quintet- that I've so far heard- for he was one of Miaskovski's best
pupils, and considering the competition in that category- Kabalevsky,
Khachaturian, Shchedrin and others- that's .saying. something), and other
pieces besides...  (apparently the 2nd piano concerto of Golubev was also
recorded, contrary to my recollection- my mistake)

-Eric Schissel, rambling a bit. Sorry.