Are you by any chance referring to the video in this thread ? :
Robert Babicz video on mastering (and other stuff)
In this video, Robert mentions that he usually cuts off everything above 17 or 18 kHz.
I found this a bit odd and it's not considered common practice in mastering, not by default at least.
From the times i decide to take out some high frequencies, i may use a bell (any Q), a shelve or any combination. Very seldom a hi cut-off gets used over here, or it must be to feed a compressor side-chain to make it work on the lows for a change
Peter van't Riet
FineTune Mastering