I've got a poorly indexed query and was attempting a quick work around in production by increasing work_mem when it was called. EXPLAIN ANALYZE is telling me this:
Only the rounding issue was removed in 9.3 so that it could use the full 1GB, (rather than getting to 512MB plus a few bytes, and then deciding it didn't have room to double). The 1G limit itself was not removed until 9.4.
Note that these limits were not on the total amount of data being sorted, but on the size of the array of row headers, and so limits the number of rows, regardless of the size of the rows.