ssp_libraries_ext.js file used to be unlocked earlier (SC 2019.2) and we could make direct changes to our backend code via that file, but in the new version (SC 2020.1), it is locked is this by design in the new version or can we unlock it somehow?

This library file should never be unlocked for SuiteCommerce. It is also a generated file, which is created from all of the SuiteScript you include in your extensions. If you want to add new code to it, you should put it in an extension and then activate that extension.