Open the Facebook app on your iPhone
Tap your profile icon (bottom-right)
Tap Settings and privacy
Tap Settings
Tap Accounts Center
Tap Personal details
Tap Account ownership and control
Tap Deactivation or deletion
Choose Deactivate account
Select a reason, then tap Continue
Enter your Facebook password, then tap Continue
Tap Deactivate to confirm
Open Settings on your iPhone
Tap Notifications
Find Facebook and turn off Allow Notifications
(Optional) Go to Settings → General → iPhone Storage → Facebook → Offload App or Delete App