Looks like UXP on Vista / XP will live as long as roytam1 remains among the living. From user's perspective, I'd say it's approximately as good as support for Linux from official offerings, both have shitty WebGL performance and non-existent support for hardware-accelerated video decoding. XP struggles with some fonts' display if site goes too much outside of web safe font territory.
As for running the forks on modern Windows...well who can tell the difference between an app that uses SRW locks, GetTickCount64 and an app that uses critical sections, GetTickCount and whatever other differences under the hood? Though Ideally, it's probably the best to prefer best available methods available on certain OS.
Interestingly, Serpent also comes with early implementation of web extension support. Back in late 2023, I got KeePassXC-Browser working with some limitations, for filling passwords from KeePass (using KeePassNatMsg plugin), including One-Time Passcodes (no Pale Moon compatible extension can do the latter bit). Still works today.
Original KeePass uses recent version of Inno Setup installer (which supports Windows 7+), but the application itself, based around .NET Framework, is quite scalable, it took many years before it broke on Windows 98 to the point of crashing on launch.