Will the “pity” rate be retained even after the event banner is over?

Yes, according to the developers, the counter towards the guaranteed 5-star character “pity” reward will carry over. As a summary, you'll only get this consolation reward if you're unlucky enough to reach 90 wishes without ever getting a 5-star character. However, if you manage to get a 5-star character before fulfilling the 90 wishes, the counter will be reset.