The File Formats wiki seems to indicate The Unarchiver and a few other open source tools support this format:

http://fileformats.archiveteam.org/wiki/StuffIt

Edit: as does the ReStuff page actually.

The Unarchiver has been acquired and is proprietary now, I believe.

The command-line tools are still open source though:

https://theunarchiver.com/command-line

I'm not sure there is any legal basis for being able to charge a license fee for using a file format, only for a specific implementation of that format's codecs (unless it is patented, which I don't believe stuffit is or was).

Well, any patents would have expired by now, right? So how could they stop a clean-room re-implementation? (That is, one person/team describes the format, another person/team implements a decoder from the description).

Compression isn't super complicated generally. I bet with a good number of input output samples it wouldn't be hard to reverse engineer. In that case, you wouldn't even need the two teams.

Draw a Venn diagram of people who 1. have the interest to do it 2. have the skill to do it 3. have the time to do it.

It doesn't have to be one person. If 1 is strong enough and related to revenue in some way, 2/3 can be solved by throwing money at the right people.

How much revenue are you thinking there is in converting archive files from Stuffit versions 4 and 5 (released 1995-1997) to Stuffit version 3.5 (originally released 1994). As far as I know, neither has been commonly used since the release of MacOS X in 2001 (I mean, older Mac OS machines were surely being used for several years after that too, but it's been a while). It does not seem like a particularly likely revenue opportunity. Like most other archaic orphaned file formats, with a few exceptions.

I'm not totally sure who the audience is for this hobby project even! It says people running Mac OS System 6, which was released in 1988, replaced by System 7 in 1991. (If you were running System 7,or system 8 released in 97, or system 9 released in 99 -- you wouldn't need this converter). I do wonder who these people are running a 1988 Mac OS release, on what hardware, and what they are doing!

A Mac OS System 6 machine CAN connect to the internet, but barely, I think this may have been the first version that did TCP/IP built into the OS, but it was finnicky. It got a lot more solid in System 7.

TCP/IP wasn't bundled with the OS until System 7.5 (in 1994), before that it was sold separately ($60!)

thanks for correction!

This makes me think. Are there consultancy shops (or individuals) that specialize in reversing/opening file formats/protocols etc?

There's a bit of that happening in security research, so in that way, yes. A quick search suggests that they exist independently as well, for example http://www.noat.com/

Nothing to do with proprietary versus open. For instance, a Linux ext4 filesystem image cannot be mounted by the ext2 filesystem. That would be forward compatibility.

There is backward compatibility: ext4 driver mounts ext2 image.

You can have ext4 partitions that are readable by ext2 drivers, you just have to disable certain ext4 features.

But more to the point: If you want to open ext4 on a system that was written before the release of ext4 you can just implement a program that can read ext4. Meanwhile apparently the simplest way to open a StuffIt 5 file on System6 is to spin up a System7 VM on a different computer, extract the file with an installed copy of StuffIt 5, compress those files with a copy of StuffIt 3.5, then move that file from the VM to your System6 computer.