They have a blog post about the open source license they chose: GNU Affero General Public License V3 (AGPLv3).

https://plausible.io/blog/open-source-licenses

From their blog post:

> If you used AGPL-licensed code in your web service in the cloud, you are required to open source it. It basically prevents corporations that never had any intention to contribute to open source from profiting from the open source work.

> It explicitly prohibits corporations from parasitically competing with an open source project. They won’t be able to take the code, make changes to it and sell it as a competing product without contributing those changes back to the original project. [emphasise mine]

The blog post above was discussed heavily on Hacker News back in Oct 2020: https://news.ycombinator.com/item?id=24763734