The voltage when idling starts at about 14V when just started, but after about 15 minutes of driving drops to about 13.45V, also at idle. We saw it drop to about 13.0V or 12.9V on Saturday while driving. No mods connected.

The battery was deeply discharged twice a few weeks ago, down to about 10V once and below 6V the second time. Now, when resting, it's dropped from 12.5V yesterday to 12.4V today, 24 hours between readings, truck parked the whole time. No weird temperatures, 40s overnight and 60s-70s daytime.

Is it likely one or more cells in the battery are bad, and that's dragging down the alternator? Shop did a load test, says the alternator was fine for 10s @ 130A. Battery was said to be okay if a bit low.

All your tests point to everything being OK. But you should get a battery load test, not the alternator.

That's likely what the shop did, but they also said they tested the alternator as well.

I would have sworn that the 5th Gen still had a fixed voltage alternator, not a Euro 5/6 "smart" alternator. Is that not correct? My recollection is that the alternator was steady at ~13.8V until recently.

The alternator output voltage can vary depending on temperature. All your voltage measurements are OK. (Keep in mind that the temperature in the engine compartment will be much higher than the outside air).

Yes, draining a battery down can damage it and shorten it's life. Your measurement of 10 volts indicates one bad cell (2 volts/cell), and 6 volts indicates three bad cells.

A battery load test (measures CCA -- cold cranking amps) will quickly show a bad battery. What is the measured CCA of your battery?

Since the alternator test shows it putting out 130 amps, that is not the problem/issue. When alternators go bad, it is usually the diodes and you will loose one or two phases of current (alternators are three phase). When you loose a phase due to bad diodes, you will get a sound on the AM radio that varies in frequency depending on engine RPM.

Two suggestions:

1. All batteries go bad eventually. You could be preemptive and get a new battery now.
2. Buy a quality jump box and keep it around so you don't get stranded when the battery does eventually go bad.

I did #2. My OEM battery is now six years old, and still going strong, but just to be ready when it does go bad, I got this: Amazon.com

How old is the battery?

[QUOTE
I would have sworn that the 5th Gen still had a fixed voltage alternator, not a Euro 5/6 "smart" alternator. Is that not correct? My recollection is that the alternator was steady at ~13.8V until recently.[/QUOTE]

Depending on the year, some do...my 2019 is a perfect example.
The "Smart Alternator" can be made "Stupid" (a good thing) by using the "GM Diode Fix"
I dont have the thread handy, but do a search on the forum and you will run across the details. There are several different threads on this as I recall...

I did this on mine a couple of years ago and have never looked back. The alternator happily keeps both batteries fully charged with no residual issues.

I had a similar issue a few months ago with a NEW battery. It got deeply discharged (I have an always on dashcam) while I spent two weeks in IL and came back to a dead battery. It was charging, but wouldn't hold a charge. My truck was dying about 10-15 mins into a drive if I came to a red light or stop sign.
Thankfully I bought it from Costco, I told them what was going on and they swapped it out.