Comment by GregorStocks

You might be interested in Premodern: https://premodernmagic.com/. You can play it on regular old MTGO.

FOSS Magic clients are in a legal gray area at best. My mental model is that Wizards de facto tolerate clients like XMage and Forge because their UX is awful, but if you made something that's actually as user-friendly as MTGO/Arena, they'd sue you and you would lose.