"My first solar charge controller"

[paulca]

But when you turn off the buck, won't the voltage go back high?  Won't it oscillate?

It probably would yes.  I could slow that down with caps, or make it "AND" with the MCU.  The MCU says ON (knowing the voltage) AND the zenner diode is conducting.  Making the zenner just a safety cut out.

The buck turning on and off, one and off, isn't really big issue, it's basically PWM.  However, when testing a buck/boost converter at 4V it was perfectly stable.  At 3.8V it went nuts and slammed the output voltage to -32V when it was actually set to +12.  It's that kind of response I have to avoid, hence hardware lock out.

[paulca]

This tech note/white paper.
https://www.analog.com/en/technical-articles/digital-adjustment-of-dcdc-converter-output-voltage-in-portable-applications.html
The safety undervolt lock out isn't in there, might be in one of the datasheets for the buck ICs.  Anyway I'm sure it's a fairly standard voltage supervisor circuit.

Haven't read, just found this also:
https://www.ijert.org/research/digital-control-of-dc-dc-buck-converter-IJERTV1IS7038.pdf

Seems there are also quite a lot of master's and doctrate thesis(es?) on the topic, but with various flavours of application area.