They do have a contractual agreement, Travis said as much in IOI Insider from June 2021. They also have no plans to move it citing such agreement. I think both assumptions (exclusive to PS4 and H2016) are correct, but that is just my opinion. We will likely never know for certain.
What employee resources? They are in the files of Hitman 3 on PS4. It’s not very hard (for IOI) to change that and add them onto other platforms, they run intermediate formats that are converted to platform specific ones automatically when a resource is required to be packaged. It’s barely any work.
All escalations from Hitman 2 are included in Hitman 3, the ones from 2016 that weren’t ported are in the files but aren’t very good escalations. Only the good escalations remain from 2016, they really went overboard with them.
As thrison rightly said, they were in the files for the PS4 version of Hitman 3, they’re also in the PS4 version of Hitman 2. But it’s pretty trivial for IOI to move them onto other platforms.
All in all, the only work they’d have to do is add the contracts, campaign, and challenges to the servers, which also isn’t that hard. Hell, they’re doing that every update with ETAs. The missions themselves are pretty much just elusive targets though if we’re being honest.