Mullvad is fully open source, with the source code provided here [1], which has also undergone multiple rounds of audits with the reports available to the public [2][3].
It's a shame the API isn't open though. I maintain a Terraform provider for it, but it has to come with a fat warning that it can break due to (reversed) API changes, and that fixing it may require breaking changes or even not be feasible etc.
[1] https://github.com/mullvad
[2] https://mullvad.net/en/blog/2021/1/20/no-pii-or-privacy-leak...
[3] https://cure53.de/pentest-report_mullvad_2021_v1.pdf