I've been working with a client for a few months now and it's been okay. They recently hired a new "editor" and have since "restructured" their model.
The new copy standards are entirely different from the original and include black hat practices (SEO stuffing) which don't read well at all. They also state they will not pay for assignments that need more than one revision.
This seems pretty shady and puts them in a position to reject assignments, not pay, edit the work to their satisfaction, and continue using them. I've asked other freelance writer friends and they've said "No way, drop them".
In my 10 years of copywriting, I've never experienced this. I intend to end the contract but am asking if they are allowed to do this on Upwork.
It doesn't really matter if they are "allowed" to do that (contracts evolve over time, They are allowed to ask for a different type of work and you are allowed to decline and end the contract.
Freelancers can close a contract at any time.
Of course you can close a contract on a client who starts asking you to do "black hat", naughty or otherwise unseemly things.
Of course you can close a contract on a client who starts doing goofy things like saying they won't pay you for your work (!!).
(In fact: You can close a contract for no reason at all. You don't need to justify doing so.)
If I client did that to me... I would still want to end things as amicably as possible. I'm not their pastor or rabbi. So I probably wouldn't waste my time trying to teach them all about the things they are doing wrong. I would probably just try to end things quickly while still retaining their good favor and hopefully positive feedback.
As for your question: Are they allowed to do this on Upwork? I don't know every detail of what they are doing. it sounds like bad management which is only going to bite them in the end. And it sounds unprofessional and unethical. But I can't think of anything you said that makes me think they are violating Upwork ToS. So I would not report them to Upwork or anything.