I believe it to be safe, and the warning has an easy menu to add it as an exception, but I wanted to make you aware in case you want to get it published so that it is recognized in the future properly. It will also likely have increased use / membership since it will be more easily recognized as safe. Most of the games I have used on itch.io do not give that warning so I assume they are registering the other games' software somehow with Microsoft's list of known publishers.
I pasted the image so you can see what message people will get.