In Canada (on many mortgages) you are only allowed to prepay a percentage per year, if you pay off your mortgage in a shorter window the financer claws back interest they would have made within that term.

> Some web searching suggests that only about 2% of home mortgages have prepayment penalty clauses.

It's clear (to me) that you're talking about the US specifically, but it might not be clear to everyone.

Residential mortgages are highly idiosyncratic to the country you're talking about. Try getting a 30 year fixed rate mortgage in the UK.

is there somewhere where a majority or a plurality of mortgages come with a prepayment penalty?

The UK.

It's standard in the UK and probably other parts of the world.