Actually 18 months.
Did you actually read the announcement?
Say what you will about what they are doing, but at least they are giving people plenty of time to get their data.
(Or did you mean something else with "clear that space"?)
Examples for things that will break once your account is read-only:
* Any program that modifies files on open (Word, Excel, others)
* Any program or device that would sync to a cloud folder on OneDrive
* Any existing workflow that relied on OneDrive files for easy access and/or sharing
Have you ever mounted a filesystem read-only? You should try it and then come back and tell us that people have plenty of time. They might not lose their data immediately, but virtually anything that touches OneDrive will break once it becomes read-only.
Your wording suggested the data backup was at risk. You can still rely on the safety of the data backup, they're doing fine there, even after 90 days. Just don't expect to be able to add additional data, which will of course break workflows trying to do that. The data is safe though.