iOS 15 on the iPhone: two functions that you should definitely switch off
As a rule, iOS updates run cleanly and there is little that would be classified as "critical". With iOS 15, however, Apple has a new option (and left an old one), which you better disable in most cases, otherwise your mobile data volume will be used up in seconds.
Disable WiFi support
An older feature, but still active for some people and consuming mobile data, is the Wi-Fi Assist feature. The feature is actually well thought out, because it ensures that the iPhone switches to the mobile network if the WLAN connection is poor, so you can benefit from the better Internet speed.
However, if you don't have a data flat rate or didn't bring a large data package with you in your mobile phone contract, it can quickly happen that you use up your entire data quota for the month - without noticing it. Actually, one thinks that one is always on the go at home in the WLAN, but with this option one can quickly switch to the mobile network, just because one is just turning on the washing machine in the basement.
My recommendation is therefore to disable the "WLAN support" option. The setting for this can be found under Settings > Cellular > WLAN support (note, the option is at the bottom of the cellphone page and you first have to scroll past all installed apps!).
Turn off mobile data backup
The second function, which is also only available for 5G iPhone models (iPhone 12 and higher), is called "Backup via mobile data". It's not difficult to guess what's behind it: the iPhone always creates backups from time to time and usually only sends them to the Apple cloud when the device is logged on to the WLAN.
With iOS 15, the option has now been added that the iPhone can also send a backup to the iCloud via the mobile data network. This can be useful if you don't have to pay attention to your data consumption, but if you have a limited data package per month - like most people in Germany - you should disable the option.
Depending on the storage space on your iPhone , a complete iCloud backup can quickly be 20 or more gigabytes, which is more than enough for most mobile phone contracts. Once the backup is in the cloud, the iPhone only sends up smaller amounts of data because it only updates changes to the database, but even then, a gigabyte can quickly accumulate every day.
Under Settings → Your name → iCloud → iCloud backup → Backup via mobile data you will probably find a green switch. You should deactivate this (set it to gray) so that no data is consumed by iCloud backups in the future.
It is also incomprehensible at this point why Apple activates this option by default. In the descriptive text below the function, you write yourself that activating this option can mean that the data volume can be used up faster than usual.