How can I delete a subscription on my iPhone?

If you don't want to continue a subscription you need always to cancel that subscription in time, see the following Apple articles:

If you want to cancel a subscription from Apple

If you want to cancel a subscription from Apple - Apple Support

If the subscription that you want to cancel is not listed

If the subscription that you want to cancel is not listed – Apple Support (UK)

If the subscription is expired no longer active you can't remove its, it'll stay there for a long while