For this reason I never use my main account for such kind of registrations. For less important registrations, especially in crypto sphere, I have dedicated email. I'm ready that I will get lot od spam messages to this email address. So, I don't care much if somebody exposes my email address in public.
I don't think that somebodt can force these bounty managers to not show participants emails in public.
A bit off-topic - I received some ICO advertisements, message about selling bitcointalk email database to email address which never was used for crypto related registrations, I have no idea where did it come from.