One reason "shy Trump voters" doesn't make as much sense as you'd think as an explanation for polling errors this year is that Republican candidates for Congress generally outperformed their polls by more than Trump did.

"It's harder to reach certain types of voters on the phone, including in ways that are hard to solve with demographic weighting" has always been a more robust explanation than "people are too ashamed to admit they like Trump".

Anyway, a good lukewarm take is that declining response rates to polls—long cited as a reason for concern—are starting to take their toll, but not in a way that will be especially predictable. Indeed if the effects were predictable, they would be easy to solve.