Things changed a little: "The project was officially incorporated as a gGmbH (a German form of non-profit LLC) in 2021 but lost its non-profit status when Germany removed software projects from eligibility. In 2025, the team began the process of moving the project to a new foundation, and began implementing community governance."—https://joinmastodon.org/about.
It's now "Mastodon, Inc. is a 501(c)(3) non-profit entity in the US that supports the growth and operational capabilities of Mastodon"