For example, Open Watcom is nonfree because its license does not allow making a modified version and using it privately. Fortunately, few programs use such licenses.
Although the FSF doesn't like licenses that force release of code of private versions, it should be noted that Open Watcom also has a termination clause. You can no longer use that software if you are being sued by Watcom or something like that.
This termination clause is why entities who otherwise would be okay with this license, like Debian, don't find it acceptable.
Proxmox is unique from other projects, in it's much more hacky, and much of the stack is custom rather than standards. Like for example: For networking, they maintain a fork of the Linux's older networking stack, called
ifupdown2
, whereas similar projects, like openstack, or Incus, use either the standard Linux kernel networking, or a project calledopenvswitch
.I think Proxmox is definitely secure enough, but I don't know if I would really trust it for higher value usecases due to some of their stack being custom, rather than standard and mantained by the wider community.
If you're interested in deploying a hypervisor on top of an existing operating system, I recommend looking into Incus or Openstack. They have packages/deployments than can be done on Debian or Red Hat distros, and I would argue that they are designed in a more secure manner (since they include multi tenancy) than Proxmox. In addition to that, they also use standard tooling for networking, like both can use Linux Bridge (in-kernel networking) for networking operations.
I would trust Openstack the most when it comes to security, because it is designed to be used as a public cloud, like having your own AWS, and it is deployed with components publicly accessible in the real world.