Subsidized by Automattic, the creators of Wordpress.com and the (large, 790+ employee) company that is main commercial sponsor of Wordpress open source project. https://automattic.com/

The iOS, Android, and Electron/desktop app is open source, but the server-side environment is proprietary to Automattic. But, the app supports simple zip export of all your notes, which is nothing more than a directory full of plain text files and a JSON file representing the (optional) revision history. I believe the Automattic team operates the service for free because they use it as a testing ground for development frameworks that then cross over to Wordpress.com and other projects.

I received few marketing emails related to a topic that I had mentioned only in a Simplenote note and also started seeing online ads for few days. It might just be coincidental but since then I have moved to Notes.app temporarily and I am looking for a replacement - a third party minimal and private note service.