A few weeks ago, I received a heads-up from Prof. 9 on Twitter about something peculiar: the Mega Man helmet available to purchase for your Xbox Avatar was no longer in the Xbox Live Marketplace. Upon further investigation, I discovered that it wasn't just the helmet -- all of the Mega Man costumes, accessories, and related fun stuff for Avatars were no longer available there.
This seemed like an odd move, so I investigated further. It took a bit of time, but a Capcom representative told me the following:
“These avatars were published by a partner for a limited time period, which has now expired. Apologies for any confusion or inconvenience.”
So that's the long and the short of it: the accessories weren't from Capcom, but were licensed through another company. With the Xbox 360's days waning in favor of the new Xbox One, which has a lessened emphasis on these characters, it's easy to see why the company might not want to renew the license -- especially if they were no longer bringing in money.
As far as I'm aware, the standard protocol for this should still allow those who purchased these items free access to use and redownload them as necessary, but those who did not buy them before will no longer be able to. I could be mistaken on this particular matter, but that's by and large how these digital purchases work when the license is up, though there are some rare, extreme exceptions.
For a look at the full ensemble as it was, you can check out Brelston's old post from a few years ago on Capcom Unity.