As detailed in Apple’s support document, Gatekeeper is getting stricter about running binaries that haven’t been notarized. I imagine warnings will get progressively scarier from here on out.
The new requirements appear in 10.14.5, currently in beta. Aura isn’t currently notarized, just signed with a Developer ID certificate. I’ll be releasing a point release for Aura v4 (build 10400) which adds the hardened runtime and notarizes the app.