I've noticed that in-game Project:Red give a message that there's an update available.
However, this update (4.3.7) is marked red which means: "Red builds can be unstable. Very unstable.".

Will there be an option to disable the update-message for unstable builds or in it's entirety?

One thing you can do: changing the update check address for new versions in order to old versions see an empty changelog, so they will display nothing in the chat.

This is for 1.7.2, we need an option for 1.6.4

Not updating 1.6 anymore.

Then how do you stop 1.6.4 users being spammed with update notifications for 1.7.2 builds?

1. Update to 1.7.2
2. Ignore it, it lasts a few seconds, and only shows ONCE per game load

1. does not currently work for mod packs because they can't update, so please just go with 2) like I do.

See 3b05b44