I’m also very interested in this feature.
For a number of the websites that I use Bitwarden to auto-fill extra form data for, the identifiers that I am required to use in Bitwarden to identify the extra field are, at best, ugly. At worse, they are completely random characters that tell me nothing about what the field actually is for.
It would be awesome to have a third field available to give a ‘human-readable’ name to custom fields!
I also wanted to link this related feature request: Allow separate HTML name for custom fields.