We also have various windows tags as well as e.g. macosx, kali or debian although we do not support questions that are specific to those foreign operating systems.
Their main purpose are dual-boot questions where there are issues having both Ubuntu or one of its official flavours installed together, but other common use cases include questions about networks with Ubuntu and foreign systems, migrating from one system to another, converting data in OS specific formats (like binary packages), emulation and virtual machines.
You see, while these tags for themselves might suggest that a question is off-topic, this is not necessarily true. The world of operating systems is not strictly divided, there are many points where different systems interact in any way with each other. These tags help to categorize such cases.
Of course I agree that we often get off-topic questions that are only about the foreign OS, but then the tags can at least help us to spot and handle them efficiently as well.
TL;DR: Keep this tag!