I couldn't fit a better explanation in the title, it affects all users who don't upload a custom profile picture, not just new accounts. A lot of people, myself included, just use(d) the default Identicon[1] (random-ish generated pixel art, basically). The new avatar is a grey rounded GitHub logo.

I noticed this just now after restarting VSCode and seeing my avatar look different. On the website, the top right account button still uses the Identicon for me, but changing any URL parameter of the image shows the new generic avatar, so I suspect it's just cached on their CDN.

I found a discussion[2] on GitHub about this, some people in there see it as a sign of an impending ban, I highly doubt this is the case here, since browsing any large-ish list of users anywhere on the website shows no Identicons and a lot of the new generic profile picture.

The GitHub changelog[3] doesn't list anything (yet).

[1] More context about what Identicons are: https://github.blog/news-insights/company-news/identicons/

[2] https://github.com/orgs/community/discussions/211045

[3] https://github.blog/changelog/

Relevant Reddit thread: https://www.reddit.com/r/github/comments/1xcxbup/github_profile_picture_reset/