Disable full immersion permanently

If you mean the Focus feature, you cannot remove it from iOS.

There is nothing, however, forcing you to create custom Focus modes or to switch between the predefined ones.

Set up a Focus on iPhone - Apple Support

“Full Immersion” is not a standard iPhone setting name in English. You may be referring to “Full Immersion” in another language, or perhaps “Focus” / “Do Not Disturb” / an immersive display mode.

If you tell me where you see “Full Immersion” on your iPhone (or send a screenshot), I can tell you exactly what it means and how to disable it permanently.

To improve my understanding of the phenomenon, it would be highly beneficial if you could share a full screenshot (Do not crop it). Visual representations carry a greater impact than written descriptions.

Please remember that this is a public forum, so when attaching a screenshot, please avoid including any personal credentials such as IP addresses, card details, email IDs, Apple IDs, IMEI numbers, serial numbers, phone numbers, order IDs, or any identifiable location information if you are sharing a map.

With the limited information that you have provided, It is hard to even guess the problems faced by you. 

You may get faster and better responses to your query by --> Writing an effective Apple Support Commun… - Apple ...

Please re-post with detail so that someone else can help.