The CSI requirement comes from EU directives that apply to all member states.

It was a requirement as early as 1990, it was in place in the EEA Regulations 2000, and even the Immigration (European Economic Area) Order 1994.

This means that, if you apply for PR without it relying on periods when you needed it (periods spent as a student or a self-sufficient person), then your PR application would be refused, regardless of how long ago this period was.

All PR applicants relying on periods spent as students or self-sufficient persons must have proof they were covered by CSI, even when their qualifying period is before 2011.

There was a very specific exemption before 20th June 2011, for students who had been issued an EEA registration Certificate as a student before 20th June 2011. But this is the ONLY exception.

This is documented here: UK Government Web Archive