ClassiCube

Heres a tutorial for joining servers with outdated clients:

First find the details in the direct link for the server you wish to join by going to https://www.classicube.net/server/list/ and copying and pasting the direct link to a text document (make sure your logged in as your desired account first)

Example of what you are looking for: mc://<ip>:<port>/<username>/<mppass>

then:-

if your using Command Prompt type: ClassicalSharp <username> <mppass> <ip> <port>

if your using Windows Powershell type: ./ClassicalSharp <username> <mppass> <ip> <port>

Hope you have find this tutorial helpful in using older clients if you wish to.