They bought the rights to a game that I previously purchased on Steam (Rocket League), then promptly dropped Linux and Mac support and moved everything to the Epic Games Store.
Have to confirm, but pretty sure I couldn't install it via steam on a fresh computer. Same account, play on original computer still, etc.
A fresh computer won't matter. You still have the license on Steam. It would only be a problem if trying to play on an unsupported platform, as is the case with Linux.