It is entirely discretionary on their part.
CRA policy instructs them NOT to do so, as the CRAs want to preserve as much prior credit history as possible.
All you can do is ask.
"Letting it fall off" only removes the ability of others to become aware of the unpaid debt by seeing a charge-off (or collection) with no update to show that it was paid.
It does not discharge the debt.
Whether others may still become aware of the unpaid debt or make its payment a condition for any new approval of credit is an unknown.
Paying removes that unknown.
