Is Proxmox really free?

Yes. Proxmox VE is free to download and use, and the full platform is open source under the GNU AGPL v3 license. You can run virtualization, clustering, backups, and storage features without buying a license key.

Paid subscriptions are optional. They give access to the enterprise repository (more strictly validated updates) and official support. If you use the no-subscription repository, you still keep the same core functionality, but support is community-driven.

That model is one reason Proxmox is popular with both home labs and companies trying to control virtualization spend.