When you visit avatars.duolingo.com, you get a 404 error. That means there is no website to use or trust. The domain is a subdomain of Duolingo, the well-known language learning platform, which is a positive sign. But a legitimate service should actually serve content.
The site has no privacy policy, no contact information, and no record in the Wayback Machine. It does use a valid SSL certificate and is hosted on AWS CloudFront, which are good technical indicators. But none of that matters if the site itself doesn't exist.
If you're wondering whether avatars.duolingo.com is a scam, there is no evidence of malicious intent. But it's also not a functioning website. The most likely explanation is that this is a deprecated or misconfigured subdomain that once hosted user avatar images. Until the 404 is resolved, treat it as an unavailable service and don't expect it to work.