@Edent I think you can do the same trick with a webfinger endpoint:
curl -H "Accept: image/avif" https://shkspr.mobi/.well-known/webfinger?resource=acct:whatever@shkspr.mobi&rel=https://webfinger.net/rel/#avatarrel query parameter is already defined in the spec: https://datatracker.ietf.org/doc/html/rfc7033#section-4.3
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