That's not about a hack or something like that,
that's just about 126.96.36.199, for example GOG or an official version, like when you install it from the original CD and then install it's patch 2.02.
Just tell them to use 2020 (combat or any other 2020 client), or play with them as is. Just remember they use 2021.
Worked fine all these years. And that is just a notification.