There are two amalgamations. One is the “pretend” amalgamation in the Juce distribution, which is not a true self contained amalgamation. The other is the amalgamation that I have put together using my modified version of the old Juce amalgamation tool. This true amalgamation is here:
You asked about the amalgamation staying “up to date”. I do plan on periodically recreating the amalgamation to reflect Jules changes in the master branch. Say, a couple of times a month. When Jules puts out tagged version of Juce I will definitely recreate the amalgamation, and tag it with the same label in my own repo. So you can always refer to a tagged amalgamation.
This having been said, unless you are picking up a new feature that you need, or there is a bugfix that you must obtain in order to move forward, there is no real disadvantage to having Juce sources that are a little out of date.
Now if you want to rebuild the amalgamation yourself, it is pretty straightforward, everything for doing so is in the “Source” directory of the JuceAmalgam repo. You also need to pick up my amalgamation tool:
You would also need to clone the Juce repo and make its directory a sibling of the Amalgamate tool directory (which should also be a sibling of the JuceAmalgam directory).