As part of a Document Management or Intranet project I have often been asked to add a new property to the User Profile Properties.
Some of the more common purposes are:
Employee Number
Initials
Team (subdivision of Department)
It seems to be a fairly common request as questions about how to enable searching on those new properties occur often in the PnP Modern Search Web Parts GitHub project.
I have used these links as a reference for making a new User Property searchable:
How to create searchable user profile properties in SharePoint Online | RLV Blog (rlvision.com)
Warning: patience is required when working with Search
