On the upside, it's multi-platform (Linux and windows) FLOSS under active development.
Heimer is a desktop application for creating mind maps and other suitable diagrams. It's written in Qt and targeted for Linux and Windows.
Here's the official introductio video:
Heimer latest Windows release (currently v1.16.0; dated 2020-04-17) can be downloaded from https://github.com/juzzlin/Heimer/releases.