tl;dr: Apple is moving Sign in with Apple and Hide My Email aliases to a new `@private.icloud.com` subdomain, making it trivial for services to block all relay addresses without affecting regular iCloud mail users. This undermines the privacy value of Hide My Email, since previously the aliases were harder to distinguish from real iCloud accounts. Users wanting to preserve existing `@icloud.com` aliases should generate them before the change takes effect (rate-limited to ~30/hour).