Where is the current version's open source code at? A look at sourceforge only showed code from 2016.

There should be two dedicated pages (one on a website, the other in the official source control repo) of the following:
Signing public keys for verification
List of sha256 signatures for downloads


This would improve upon what happened as mentioned in your blog: Important Security Announcement to Our Valued Users Sep 13, 2023

It'd also be nice to have a BUILDING.md in the source code that makes it easy to compile and build the tool myself.

This page claims the software is open source, but I don't see the source code for the current download versions.

https://www.freedownloadmanager.org/features.htm

Hello,
FDM6 is not an open source project. It was a mistake on our web site.