The last patch doesn't allow mouse look since I messed with the new aim assist. I can no longer play

Warhammer 40,000: Darktide Bugs
2

I’ve raised this for investigation, but in the mean time you may be able to “unbreak” it by wiping your AppData. This will clear any custom options including your keybindings:

  1. Press the Windows key + R
  2. Enter %appdata% within the search input and select ‘OK’
  3. Navigate to AppData\Roaming\Fatshark
  4. Delete the entire Darktide directory

If this doesn’t work, see whether Steam Cloud Save is interfering by auto-restoring it. This option is accessible by right-clicking Darktide in your Steam library, selecting ‘Properties…’ and see the ‘Steam Cloud’ option within the ‘General’ tab.