Replacement for low resistance JFET current limiter?
You can probably use depletion-mode MOSFETs, though 5mA may require some added resistance (to have more negative gate voltage for a given current), that's pretty low for Idss.
Edit: A pair of LND150/250 would seem to meet your requirements. Maybe too high resistance.
I'm pretty sure JFETs are not going to go completely out of use, they're still used by the boatload in electret microphones. There are also boutique suppliers of them, if cost isn't a big issue, it's very difficult to get the combination of low noise and high input impedance any other way for some kinds of instrumentation. For whatever reason(s), JFET-input op-amps do not have competitive performance. I snapped up a reel of BF862s for that reason (0.8nV/rt-Hz noise)