Unsurprisingly, Upwork expressly forbids this. "Keep contact with potential clients inside Upwork [... or risk account termination]".
https://support.upwork.com/hc/en-us/articles/211067618-Freel...
I'd say once a good match between a contractor and a client has been established, both parties should swiftly move away from the Upwork platform, their rent-seeking "rules" be damned!