In 2018, Bountysource was acquired by "CanYa", a company known for their CanYaCoin ICO. As far as I can tell since the acquisition they have only gotten shadier (fair or not, I now associate them with sketchy cryptocurrency operations). Would not recommend.
I wanted to find a platform to fund open source development through bounties, but Bountysource isn't it - fortunately there are new alternatives popping up, hopefully more trustworthy.
https://github.com/fossjobs/fossjobs/wiki/resources#bounties
My suspicion is that people contributing to open source don't really just contribute randomly. It is most likely that they use a library or repo and then they end up on the "issues" page because they had a problem themselves.
It's at this point where I would imagine one is most likely to convert into actually picking up an issue and when notice of a "bounty" might push someone over the fence (?)