I actually did consider things like author's notes and background material, but I stand by that not being the same as what you're describing anymore then I'd co

I actually did consider things like author's notes and background material, but I stand by that not being the same as what you're describing anymore then I'd consider preserving a software project to also have to include every mailing list any dev posted to or all communications or whatever. Masters, source code/raw assets, and so forth are still final products produced from a development process, even if technical necessity then dictates they are then "compiled" into something targeting specific mediums. Author notes, drafts, and original manuscripts, just like processes around music/movie/software production, can be interesting in their own right, but I don't think it's in the same bucket or has the same cultural implications.