By "header" I meant something akin to a
file format. Creating a PK3 through making a simple ZIP and renaming it to PK3 won't do, as true PK3s (like the ones made through SLADE) have a slightly different format.
It's not something you add manually, by the way - SLADE does it automatically when creating a new zip-like PK3 file.
Repacking files is frowned upon partly due to how repacking is often executed, and partly because it prevents recognizing the creator of some content in it easily.
When somebody reads "repacking", what one would envision (at least me lol) is, for example, a person picking different characters from different files or cherry picking characters from a character pack, then sloppily dropping them on a megapack of sorts, with little to no regard to compatibility with each other, the game, or what not.
Apart from the game having the potential to obliterate itself from existence, there's also the issue where (some) server hosts serve this file expecting people to download their massive 50mb megawad, which is, of course, unreasonable.
Some creators don't like this repacking thing because it screws with file versioning (how do you know what version of X do you have when it's smushed with many other things?) as well as making it harder or unable for players to find other works by the creator (maybe the mod didn't come with credits within because it is expected you'd look for the file name and find a link to the relevant thread).
There are also other reasons I may be omitting, such as creators expecting the repacker to
ask first before commiting to the act, but they don't.
Of course, some peeps are okay with this as long as this superwad is kept strictly
private, be it because you want an easier access to the mods you like instead of loading them one by one (even though there are several methods to do this...), or you want to easily share the mods you want to play with your friends without sending every single link/file, or eh
I may be explaining this badly, but that's basically the gist of it.
With that said, I don't speak for anybody in here, but if you're just sharing your repack with your friends so they have an easier time "installing" it, then it's generally fine, however if you plan on hosting it for the public, you'd probably want to ask every creator in there if they're fine with such amalgamation.
