Starting with version 1.46.0 (released yesterday) all Defender Client packages are being published under the @openzeppelin scope. These packages will have the same name with simply the scope added (e.g. defender-relay-client => @openzeppelin/defender-relay-client). This also applies to defender-serverless (it is now published as @openzeppelin/defender-serverless)
This change was made to harden our security practices in implementing tighter controls over publishing and package management and also prevents typosquatting.
Cheers!