If you do a simple upgrade, you still have stuffit.

Alternately if you did an archive & install you can search for all items that contain "stuff" in your archived system and replace them in the corresponding places in the new system, you still have stuffit.

Finally you can navigate the shitty stuffit website and try to find the free version of stuffit and install. Judging from the website, it looks like the company has been sold a couple of times to people who now don't have a clue. My guess is that it is now not part of the OS install because they demanded unreasonable licensing. Anyway get it before it vanishes.