I checked the fuel pressure on a mid '90s GM LT1 and it's about 15 PSI. It should be around 45 PSI I believe. What could cause that? Failing fuel pump? Obstructed fuel filter? I'm baffled.

Okay TX Grease Monkey. You say that electric fuel pumps "rarely go bad" on GM.

Is that so?

That's true on almost any brand. I would replace the fuel filter first. In over 3 years on this desk, I'm not aware of a fuel pump being the cause of any problem, even though many people have replaced them needlessly. In my own life, I've never seen an electrical fuel pump fail. I have seen mechanical fuel pumps fail, however.

Usually, the problem is a fuse, ground, relay, ECM, or ignition switch that prevents the fuel pump from working.

Uhm, I hate to admit it, but GM has had significant fuel pump failures that were related to the supplier of those pumps. That failure rate was, at the time, higher than most competitor's for the same class vehicles. Bad for GM. That problem was mostly taken care of on subsequent models.
That being said, little else other than a faulty fuel pump can contribute to low fuel pressure, other that a gross leak: either the fuel line is leaking, or one or more fuel injectors are leaking.
Both of those failures would be quickly noticeable.

Uhm, I hate to admit it, but GM has had significant fuel pump failures that were related to the supplier of those pumps. That failure rate was, at the time, higher than most competitor's for the same class vehicles. Bad for GM. That problem was mostly taken care of on subsequent models.
That being said, little else other than a faulty fuel pump can contribute to low fuel pressure, other that a gross leak: either the fuel line is leaking, or one or more fuel injectors are leaking.
Both of those failures would be quickly noticable.

As it turns out several things can contribute. Vacuum loss affecting the pressure regulator being just one of them. A plugged 'sock' at the pump is another. But probably not from 45 PSI to 15 PSI.

Vacuum loss would likely cause fuel pressure to default to maximum.
A "plugged" fuel sock?? Is that what you found?
If so, replace it then repeat the tests.
Otherwise, have you thoroughly inspected the fuel system for kinked lines or other possible damage? No apparent leaks, etc?

Replaced the pump today. Runs fine now.