This is not the forum for IBM Proton.
The VMware Photon Platform 1.x hit End of General Support on 2019-03-02 according to https://lifecycle.vmware.com/. Yes there still is an Enterprise Application Policy https://www.vmware.com/support/policies/enterprise-application.html.
There isn't a customer product SKU Photon Platform 2.x, 3.x or 4.x. Hence you cannot buy official Photon Platform support.
The open source Photon OS has nothing to do with the VMware customer products in which Photon OS is a part of.
VCSA, vSphere Replication, Workstation, vRealize Operations, and much more run on a strict Photon OS ~pipeline for that specific product.
And yes, the repo URL of Photon OS has changed. Actually when downloading older builds, tdnf expected issues must be fixed manually, see https://kb.vmware.com/s/article/81304. Have you already tried this right after deploying the 3.0rev2 ova with virtual hardware v13 (UEFI Secure Boot)?
Photon OS evolution is interesting, ~ "something else" indeed. There are different OS 3.0 appliance flavors for "Generic", "VMware hypervisor optimized", "AWS optimized", "Security hardened" and "Real Time". A major aspect always was/is security. The Security Advisories for 1.x, 2.x, 3.x may give an idea of the necessity of chain of packages to be held safe https://github.com/vmware/photon/wiki/Security-Advisories.
With each Linux kernel update trillions of packages permutations are given, in theory it's slightly less architecture specific. A slice of it reflects in Photon OS. You can watch the stream tv at https://github.com/vmware/photon/commits/dev.
The 4.0 release candidate has been launched a couple of days ago. Hence, rollforward and downgrade interoperability tests are still in progress 😕
