Mostly anything that's author is unrecognized is considered dangerous by Windows. If anything, I hands down 98.7% doubt that this fantastic game is dangerous to your computer.
Yeah, most likely either the signature expired, or it wasn't signed to begin with. This is very common for indie software - the fee to get the signature key isn't seen as worth it for a lot of indie windows devs. (I say windows bc they won't release a linux or mac version for some reason despite literally using Godot)