has anyone used custom segments for data segregation? (ex: segment is comprised of 2 values: 1 or 2 -> if segment 1 then the role filters out all data from 2 and vice versa)

I am currently in the process of doing so. Any questions about it?

Yes, how do you restrict access through the global search or through creating saved searches?

Do you mean restrict access to the user to be able to see and maybe edit the custom segment record itself? If that's your question, I am not sure as I have not done that yet.

No, I mean restrict access to data like transactions & entities based on a custom segment. I've done it but it only works partially