They designed a kind of "GrapheneOS", very privacy focused mobile OS (with some specific and interesting features), but their business model is a bit frustrating : the OS is free, but you have to pay monthly to use their core apps (mail, messages, etc...)
The OS is interesting, but their pre-installed core apps offer nothing special versus the market, and if you're already a Proton customer, it would be stupid to pay for apps/features already covered by Proton suite you're already paying for !
Both companies are Swiss and privacy-focused... so a merger between the two ones would be so powerful : imagine a "ProtonOS", with Proton suite pre-installed, it would be an innovative, powerful and complementary user acquisition lever for Proton, with a relatively cost-effective acquisition, with all the hard work (on the OS) already done.