I've noticed this. I remember the "good old days" when you could volunteer to be bumped for a free round-trip ticket and occasionally an upgrade to 1st class on the next flight. Then those started to disappear and the vouchers got popular. Now those are disappearing fast, too. It's been awhile since I've seen a message asking people to bump.
I've traveled alot this year for business and there weren't too many times they asked for volunteers. Even then it was only for 1 or 2. I remember in the past where they would be looking for 10 people to volunteer.