Can you use JSON for any SQL Server compatiblity level?

It has got to be compatibility level 130 or higher. It will fail otherwise.
You may change the compatibility level and then you can use JSON.

Read here:
https://hodentekmsss.blogspot.com/2016/11/change-compatibility-level-of-database.html

Compatibility level can be changed in SQL Server Management Studio.

SQL Server 2017: The compatibility level for Northwind (SqlServer 2008(100) restored from a backup file from CodePlex site is the following:

Change compatibility level using drop-down in the above.