Epic has been trying to move on for years, or so a recruiter told me half a decade ago. Their job posting now describes “Using leading-edge technologies and languages like JS, TS, and C#”, though I wouldn’t put a bait and switch past them.
MEDITECH, maybe others, had a proprietary operating system too. (We had a special deal to sell them hardware without an OS at a time when they were normally bundled.) Eventually they moved on to NT.