I had a Linux ICQ client in 1997/1998.

With the built-in option to “hijack account”, when sending a password longer than 8 characters would log you in, but official clients capped you at 8 characters.

I had to recompile some cmd line one from the linux sources to work from my home directory on my universities unix system ~2000. Back before everyone in college had laptops.

This was the 90s, so I guess reverse engineering the protocols was simple. There were even Amiga clients of it, like stricq.

It was the OSCAR protocol: https://en.wikipedia.org/wiki/OSCAR_protocol