I too get this warning with a lot of the games on itch.io , I've yet to get a hit from a virus scan on them though. Are you getting Windows SmartScreen - Unknown Publisher popup? It is good to be wary on the mean-ole-internet, and even on itch.io, but I haven't had any trouble with things I've  purchased on here beyond buggy games crashing.

I'm personally confident that this game isn't hiding anything nasty, but you can research Windows Smartscreen + Unknown Publisher popups yourself on a search engine and find out what is up yourself.

There are a bunch of hits like this: https://itch.io/t/706326/windows-smartscreen-unknown-publisher for example.

Hope you figure it out to your satisfaction!

Edit: Also if you're getting a different warning thatn the Unknown Publisher one that might be a whole other kettle of fish though, sorry if I might have jumped the gun in my reply!