It gets damaged in Return of the Jedi during the Sail barge battle, at least the flesh part is burnt. Luke covers it with a glove for the rest of the film, and of course is reminded of it when he chops off Vader's in one of the saga's most critical moments.
The glove is still present a few years later in Book of Boba Fett and the Mandalorian.
In the sequel trilogy, Luke of course no longer wears the glove and there's no flesh on the hand either. If you look real close you can see the blaster burn on the mechanical bits too. Luke also is gloveless/fleshless in the Ben Solo flashback, but not in the similar-looking force projection, which uses a big brown glove (Maybe Luke's projection couldn't recreate the mechanical parts?).
This is also left a bit ambiguous with the force ghost in ROS which never shows the hand on that side.
So why not bother getting a spare/fixing the fake flesh etc which presumabely the Rebels/New Republic would gladly oblige despite their other faults? Luke kind of keeping a reminder of a key part of his life?
Suppose it's also possible he could've gotten a clone hand too-obviously hard to do in a movement like the Rebels but probably easy to do in the New Republic era, although it's possible clone tech is outlawed (Indeed, apart from projects like Wayland and Exegol, the Empire stops using the clonetroopers and pretty much destroys Kamino).