If you have been one of the PC gamers who is still playing Halo Combat Evolved on the PC you will be rewarded for your efforts. Those that have an authenticated key for Combat Evolved on the PC and have used it before May 26th will get the special reward flair.
To prevent key generators and folks from cheating, Bungie picked May 26th as the date. Because of the date, you can’t just go out and buy it right now and use it; but if you were lucky enough to have played before May 26th you should get some flair for your armor. Right now, we have no clue what the flair looks like, but we are told it is cool.