Despite the closing of the Winter Bash last week, there are still a few hats on the profile pictures of some users. Is it a bug?
On the profile you provided1, right clicking on the avatar and selecting "Open image in new tab" returns what seems to be a screenshot:
My guess is that every such profile you will encounter will have used a screenshot of their hats (or edited one on their avatar).
When Winter Bash was still here, hats weren't part of the avatar, but were a SVG layer added upon them.
1 This is an Internet Archive screenshot of said profile when it still had the "hat", while Winter Bash had ended. Said user has now gone back to their "usual" avatar (see comments).