Deleting data is a good practice in my eyes. The majority of notes become less valuable over time.

So why should we keep these notes on main relays especially when there relays operate altruistic.

When you want to save your notes and stuff the best thing is to run your own relay and backup the things from you and some specific people.

Saving things on the client side is a bad practice because over time you will lock yourself in to a specific client which has your data stored.