Is mastercomfig releases free?

Yes. mastercomfig releases is released under Community / External hub and costs nothing. Neither it nor Team Fortress 2 has any paid content.

What do I need to run mastercomfig releases?

Team Fortress 2, which is itself free. mastercomfig releases is an add-on and will not run standalone.

The PlayBound Launcher installs it in one click from Team Fortress 2's mods list. You can also install it by hand — full steps are on this page.

Roughly 15 MB, on top of the base Team Fortress 2 installation.

Yes. The source is published at github.com/mastercoms/mastercomfig under Community / External hub.

mastercomfig releases — Free Team Fortress 2 Mod · PlayBound