Is Cloud Security Posture Management (CSPM) available for all Azure Subscriptions?

Yes, it is.

One of Microsoft Defender for Cloud's main pillars for cloud security is Cloud Security Posture Management (CSPM). CSPM provides you with hardening guidance that helps you efficiently and effectively improve your security. CSPM also gives you visibility into your current security situation.

Defender for Cloud continually assesses your resources, subscriptions, and organization for security issues and shows your security posture in secure score, an aggregated score of the security findings that tells you, at a glance, your current security situation: the higher the score, the lower the identified risk level.