1995 Toyota T-100

1995 T-100 truck 5-speed manual 100,000 miles. Took for 30K check-up a month ago and driven very little since then but… Several times, turned the ignition key and got no sound whatsoever. Eventually started robustly. Several times, while driving down the highway, clutch popped into neutral. Driver appeared to notice it was slightly harder to get into first gear.

Several problems from the sound of things.
No starter operation could be a faulty or out of adjustment clutch safety switch or a faulty ignition switch.

Harder to shift into gear could be a worn/dragging clutch or an out of adjustment clutch. (clutch pedal operation could have an affect on the above mentioned clutch switch)

Popping out of gear could be more serious. You did not mention which gears it’s popping out of but many times this is caused by wear in the synchronizer assembly, worn shift fork, or both.
Often this wear is caused by resting one’s hand on the gearshift lever; a habit which many drivers have.
Any chance the gears in question are 3/4?