Serialism was a fashion throughout 1950's. But when fashion dissapears, results of pure quality remain. Most of composers maintained dodecaphonic organisation of pitches later throughout 20th century but not other parameters to create purely serial works. And lots of them abandoned atonality from 1970's onwards. Peter Maxwell Davies writes in neoromantic style, so does Penderecki. Lutoslawski started to create ear-pleasant music in 1980's and used 12-tone dodecaphony less and less (although it never fully dissapeared - Lutoslawski is a rare true master of dodecaphony). Rautavaara made a huge turn in late 1960's to favour Romanticism - try his first piano concerto and cello concerto. Sallinen also became a Romantic composer - check his operas and symphonies. Also Joonas Kokkonen. John Adams used dodecaphony or a close-up to it as a style imitation (Chamber symphony, Harmonielehre, which is mostly eleventone music, not twelvetone) but it never became his frequent approach. Experimental composers (Messiaen, Cage, Globokar, Carter, Brown, also Stockhausen) began to explore other stuff: silence, aleatorics, theatre, metric problems and expansions, space. No dodecaphony nor serialism.
So again, with big letters so everyb(lo)ody will undestand: NAME SOME SIGNIFICANT COMPOSERS WHO HAVE BEEN WRITING SERIAL MUSIC IN LAST 30-40 YEARS, PLEASE. COMPOSERS OF REAL IMPORTANCE! THANKS!