Hi!
There is no "SRM", at least provided by Proxmox. But you get internal backups, or you can use PBS. But all that is more about replacing the Veeam part, than the SRM. However, by using PBS, and adding a layer of backup for the hosts themselves, you can easily have a setup that provides DR that way, using a PBS remote for example, and automating restore in case of a site destruction..
I do not know of a third party open source solution for that, and Proxmox has PBS, which is awesome, but will require a bit of planning and automating to make it actually useful for that scenario!
Cheers,
Gilou