At my workplace I worked on US-based client projects outsourced to my employer and I'm not working under that employer anymore. These are desktop applications and I have several development-time screenshots of them. Is it OK to use such screenshots for my Upwork portfolio?
The obvious answer is no since if you had asked those people who gave you the work you would not be asking here.
You will loose reputation if anyone cries foul.
Your US client may be offended whilst looking for workers and sees his products, do you think he will give you the bid in that case. Your former employer (if you left on good terms ) is also unlikely to be pleased or give you work Prospective clients will wonder if their work will be bandied about in the future.
It is best to always be honest and ask for permission / show attribution even if not required.
If you still intend to show it consider if there is any way to mask the content.
For screenshots drop the resolution to the point it can not be deciphered or photoshop any client specific content (but if you think that makes them useless as an example then simply recreate a dummy/fictitious example
see this experience Client wants me to take a portfolio project off my website