People fields are now multi-select by default.
For some types of fields it makes sense to limit this to one person, like with Assignee.
Besides that this makes switching less work, it could also help solve Ability to assign Project Roles when converting a task to a project using a template (manually or via a rule), as that would allow to create a field per project role whilst preventing that there are more people in the field than can be filled with the project role.