Maybe this is a new feature of Windows Defender because I’ve never seen it before since I started using Windows 10 well over a year ago. Has anyone else gotten this warning? It’s possible to override if you click on “More info”.
Assuming you downloaded from https://www.shotcut.org/download/ then I’d ignore it.
If not, then I’d (re)download from https://www.shotcut.org/download/
What version of Shotcut did you try? Some recent versions had an expired code signing certificate that might be causing this. The latest version 17.09 includes a new certificate that expires in 2020. If you got this with that latest version, then it might be a result of a new download that requires a reputation to develop on the Microsoft servers:
It’s the latest version from the website: 17.09.04. I guess it might be the reputation thing.
Installed without the Smart Screen warning here.
W10 Pro 64bit.
Same OS as mine. Here are some additional characters to meet the arbitrary quota.