Siri disappeared after update

First may I ask if you received email notifications from this thread?

Re the problem, I think Siri is unavailable in all regions so

First thing I advise is to try a Forced restart 

Force restart iPhone – Apple Support (UK) 

If no better you could Backup, restore to factory, then restore from the backup:-  

If necessary set up without restoring backup and test at that point, erase again, then restore from the backup:-  

(If the phone needs attention you will have to do this anyway) 

See here for the resources you need. Allow a couple of hours: 

How to back up your iPhone, iPad, and iPod touch - Apple Support 

Restore your iPhone, iPad, or iPod to factory settings - Apple Support 

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support 

Do you mean it is not in Settings, between Wallpaper and Face ID?

Not quite

1. BACKUP first
2. Restore by erasing all data and settings
3. Restore from backup

but try the forced restart first.

Is Screen Time turned right off? If not then turn it off.

You're welcome, glad I could help.