I read this a week ago and was really surprised at the statistics! They lost 25% of new users because it didn't show up on the results! It's also great to see the co-founder focus on maintaining transparency within their company and with it's users.
The short summary is that for nearly a year, Google was hiding ProtonMail from search results for queries such as ‘secure email’ and ‘encrypted email’. This was highly suspicious because ProtonMail has long been the world’s largest encrypted email provider.