There are people out there who willingly try to scam their customers - minimum effort for maximum return - while remaining absolutely clueless.
I can believe that, caveat emptor. I do not know what Proxmox is doing in an effort to have e.g. end user verify with them that they indeed have a subscription. But by your own admission, this cannot rely on a locally (patched) JS invoked pop-up anyhow. Also consider one can run the whole thing with entirely own GUI and just make use of the API.
I won't go too much into detail but I have uncovered "rebranded" open source projects
Yes and if they were the clueless folks, they would have cluelessly applied some Github patch anyhow. What is this forum (as moderated) supporting? More of those?
Everything in proxmox is open source - so in theory you could just go ahead and modify almost everything.
It is literally using bits and pieces of equally licensed parts to its own benefit, that's the whole point of licensing like this.
But if you *claim* that you own the softare (i.e. are the developer of it), then rip out the nag screen and invoice the customer ridiculous amounts of money it's a con.
That's a legal problem for the two parties involved, nothing to do with censoring select patches just because you do not like a particular one.