You can leave Intervals.icu connected to Strava but just untick the “Download activities” box in the Strava area in /settings. If you revoke access, Intervals.icu has to delete the data. From the Strava API terms of service:
if a user revokes the authorization previously granted for your Developer Applications to access to their Strava account, you must ensure that all Personal Data pertaining to that user is deleted from your Developer Applications and related networks, systems and servers.
Note that should Strava revoke Intervals.icu API access then the data also has to be deleted.