@Steeghs, Ronald Thanks for reaching out.
Azure service bus supports TLS 1.3 AMQP support for a few regions partially (not completely all the region is supported) (West Central, East Asia, UK South and US East) as it is not completely rolled out across all the regions. Team will be slowly rolling it out across all the regions but right now there is no ETA when it would be available across all the regions. Once it is fully supported the document will be updated.
In case if you want to create the namespace with TLS 1.3 then you can reach out to us over one on one support request so we can help you to create the TLS 1.3 namespace on your subscription after validating few things and whether it would be best to create the TLS 1.3 AMQP support namespace on that particular region.