It was a shame AOL shut down this service. Fortunately, there are other people out there who have decoded the OSCAR protocol and got it to work.

Here is how I was able to get the service working again on my own hardware.

Install any dependencies

apt install go git

Do a git clone on the repository

cd /home/<username>/
git clone https://github.com/ox/aim-oscar-server

Run it

cd aim-oscar-server/scripts
./run.sh

Download your AIM 3.5 client and click Setup then replace Host with the IP address of the computer you are running the script on.

Here is a video of it working