But wouldn't that mostly be concentrated in places/areas where their votes aren't likely to matter? Having just driven through the US South in the last month, I can confidently tell you people are not in any way shy about their support for Trump. I saw more Trump signs and flags than I saw US flags.

It's also likely this works in both directions - if you support Biden, I bet you don't have a yard sign for it if you live in Mississippi.

By construction, the effect is strongest the more you're not in the majority, which also means your unspoken support is more likely to not matter on the actual outcomes of the election.