Can I play Deltarune without a Windows or Mac PC?
What platform will you play on?
As a GNU/Linux shill, I will describe the steps to get Deltarune running.
First, install WineHQ (allows Windows executables to run on Linux). Then, download the MicrosoftⓇ Windows™ copy from the game vendor's official website.
Alternatively, you could just use a good 'ol VM or just dualboot (like I do).
As of September 23 2021, Deltarune: Chapter 1 & 2 are available on Nintendo Switch, PlayStation 4, and PC/MacOS. The Steam version can be played on Linux via Proton.