Clients would be willing to spend more on Promote campaigns, if they were able to whitelist domains, instead of only retroactively blacklisting domains, once they see domains they are not interested in in their Promote campaign.

This is relevant, because there is a brand risk to being shown on eg game sites, and this is also worse traffic from a performance perspective (more likely to be misclicks). Many clients know from experience which categories of domains work well for them, and which kind of domains should be filtered out for a campaign to be interesting for them.