From Nielson Norman Group: https://www.nngroup.com/articles/required-fields/
Summary: Using an asterisk to mark required fields is an easy way to improve the usability of your forms. Only marking optional fields makes it difficult for people to fill out the form.
A common question in many of our UX Conference classes is: should you mark the required fields in a form? If most fields in the form are required, should we still mark them? (That’s a lot of marks, after all.) The short answer is yes . And I’ll spend the rest of the article explaining why.
The Temptation to Not Mark the Required Fields
Often designers feel that the having a marker for every single required field is repetitive, ugly, takes too much space, and, with longer forms, may even seem oppressive (the form requires so much from the user!). So, they usually adopt one or both of the following strategies:
(In some rare situations, they don’t do anything: they simply assume users will magically know what fields are required; if they don’t, then they will just have to deal with the resulting error.)