Many thanks @savokov for your efforts.
I can't prove this, but my assumption is not the app, but some libraries may be missing or different on Big Sur.
So the app itself wouldn't solve this issue.
If you want to give it a try, there is a build-osx.sh script in the scripts folder.
When you open a terminal and change to the
LibreCAD/scrips folder, you can run this script with
./build-osx.sh.
If everything runs well, this should result in a LibreCAD.dmg file in the LibreCAD root folder.
This package should contain everything needed to deploy it on Big Sur.
When this succeeds you can get in touch how to send it to me and I can publish it on the download sites then.
Armin
investing less than half an hour into Search function can save hours or days of waiting for a solution