Maybe sit down and go through your ICloud collection and have a thinning out session. Deleting all the photos you never really have should have stored in the first place.
Bet it it takes more than one session !
greedy icloud constantly ask for payments. Most of my pics & vids are posted on via email etc. This ipone is 5+ years old & nothing is removed by icloud.
Don't worry, your photos are still being stored on your phone. You have your phone set to back up your photos to iCloud (Apple's online photo storage system) and it is this cloud storage that is full.
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Your photos should be backed up somewhere - if your phone breaks or is lost/stolen you don't want all your photos to be lost. You have many choices - you could pay for more iCloud storage; back them up to your pc; choose another form of cloud storage such as Google Photos.