First, I changed the title, because it was
too loud.
Apple does not want you to install apps which are not certified with an accepted certificate.
As free open software LibreCAD has no certificate yet as it produces annual costs.
And we have no MacOS maintainer who already has a valid certificate, so the LibreCAD package and app are not signed.
Same apply for Windows, where situation is similar.
See Github issue
#1548 for recent discussion.
That means, it is up to you if you trust LibreCAD and if you want to follow the instructions to open/install it.
The instructions in the GitHub issue are for M1 hardware, they probably work on Intel Mac too.
Otherwise you must ask on a search portal how to open unsigned apps on Mac.
I can't say, I use Linux.
Armin
investing less than half an hour into Search function can save hours or days of waiting for a solution