There are still plenty of domainers making money parking but it's a totally different game these days to make good money with parking - they purchase domains specifically for their parking potential and not so much the resale potential. Those could be domains that used to be previously developed and still have lots of traffic from backlinks, typo domains or domains that infringe on trademarks.
The days that you could make a nice chunk of change with PPC on a random portfolio are over. If you have domains with resale potential that could sell to end-users it wouldn't be to wise imo to prevent these buyers from reaching out to you with an offer by population the landing page with advertisements or even worse, zero-click parking.