For a naturalisation application, do I need CSI only for the past 5 years?
Naturalisation applications will request 5 years of residency evidence (unless you are married or in a civil partnership with a British citizen, in which case it is 3 years). However, the Home Office can go back 10 years to check whether you fit the ‘good character requirement’ by being lawfully resident in the UK. EU law applies and under EU law, an EU student or self-sufficient person is lawfully resident if they hold Comprehensive Sickness Insurance (CSI). You can watch a video explanation on this topic here: https://youtu.be/dZDBaO0QrHc
the3million has a separate citizenship Q&A document, with more extensive information on the CSI requirement: http://t3m.org.uk/t3m_YE_CitizenshipWebinarNov2020_QA