I’ve seen this situation play out, where the PIP was termination with X days notice. Because the manager knew, and employee should have known, the manager didn’t really work with the employee or review their work because why bother, nothing will change (outside of something exceptional happening).

Somewhat similar thing happened on the final review, but without pointing to hard data of the fact. The decision of course remained final, the manager got a light, informal water cooler “hey be more careful of optics” chat, and that was it.

Notably when the former employee tried to go a competitor, former company killed that.

For me, well, the company promised follow up with me, and that never happened. Communication was never their strong point. Another HR person had emailed me to go through standard termination stuff, COBRA, etc. "Attached to this email is a FedEx label to return your company laptop". There wasn't, so I emailed him to say so. Never heard from him again, despite three followup emails. I even forwarded the email chain to the HR person who'd been on the termination call, and they didn't reply either. So I kept the emails and printed copies in case anyone knocked on my door. I still have the laptop. I'll happily return it, as soon as they let me know how to. (I've had suggestions to just send it to their corporate address, but then I don't want to run into it being picked up by someone who has no context, and then the company says "oh, yeah, please send here").

I got a new job, fairly quickly. No-one asked about my leaving there in any depth (the joys of COVID-era business).

As for my manager, I suspect like the other reply - probably not much happened to him, other than to provide at least token optics. It's one thing for it to be a foregone conclusion, but another if somehow the ex-employee turns around and sues for termination/discrimination and it's demonstrable that the company "had it in" for the employee (i.e. setting goals and not even attempting to review efforts and then documenting that they did review).