A User object is searchable in Salesforce Classic. However the same functionality is not available in Lightning Experience (LEX).
When Chatter is disabled on Org level and Profile level:
- People User Tab is not available in Lightning
- People(Users) is missing from Global Search Bar and People is not searchable at all in LEX
In order to make the User object searchable in Lightning, the Admin needs to enable Chatter functionality.
SFDC Help Documentation: 'People' Tab is not visible in lightning when chatter is off (Org level / Profile level Chatter off