To elaborate, the security precautions you take are entirely dependent on the threat you face (or believe you face). If you're Dread Pirate Roberts, creating a detailed diary of you criminal activities is a terrible idea [1]. If you're a 13 yr old girl, creating a detailed day by day diary is probably not a life threatening decision.
You should take the precautions appropriate for your threat model and risk appetite. I suggest deleting correspondence after a set period of time. I know people that do a yearly purge of all their sensitive mail for the previous year in January.
If you are encrypting for the sake of encrypting, as some people do (see: cover traffic), then you can keep archives of your mail because you face no negative outcome if your archive is compromised. There is no downside, so go for it.
You should take the precautions appropriate for your threat model and risk appetite. I suggest deleting correspondence after a set period of time. I know people that do a yearly purge of all their sensitive mail for the previous year in January.
If you are encrypting for the sake of encrypting, as some people do (see: cover traffic), then you can keep archives of your mail because you face no negative outcome if your archive is compromised. There is no downside, so go for it.
[1] https://www.youtube.com/watch?v=pBdGOrcUEg8