Thank you! 🙂 It's all pretty simple actually.
- The characters are all simple sprites which are 'billboarded' (they face the camera at all times).
- The scenery is all using Unlit shaders with hand-drawn pixel art textures.
- The camera has a low-resolution PS1 pixel filter applied to it (called Flashback 94, although it's deprecated from the Asset Store now but you can find many alternatives including Unity's own Pixel Perfect package).
It's also important to set all the textures and sprites to be using Point filtering and no Compression in their import settings.
Hope that helps you achieve the look you want!