This page mentions the EHR used by the U.S. Department of Veterans Affairs, known as VistA[0], developed using MUMPS.

Interestingly, the source code for VistA has been obtained via FOIA requests and so the code is available in the public domain[1]. There are now forks of VistA, like OpenVista.

At one point we had to argue with Github to support syntax highlighting for M/MUMPS: https://github.com/github/linguist/pull/148

[0] https://en.wikipedia.org/wiki/VistA [1] https://en.wikipedia.org/wiki/VistA#Usage_in_non-governmenta...