blueman wrote:
Can wiring to a light off a 20A circuit be done in 14ga wire?

It will work just fine for a switched light but is technically a code
violation, even if the wire ampacity is 20A (which 14 gauge THHN-2 is.)
You could argue that you were using the 10 foot tap rule (or 25 foot, I
don't remember which would apply better here), but you'll get shot down.

What about wiring to an upstream 15A receptacle?

No. What if somebody plugs a 10A load into both halves of the duplex
receptacle, or extends the circuit someday?

Or, does all wiring on a 20A circuit breaker need to be at least
12ga?

Yes. 12 ga is not that much more expensive than 14 ga, and your time is
worth something; do the job right.

Best regards,
Bob