This is dependant on your theme/plugins so it’s impossible to answer, as s_ha_dum mentioned.
For example some themes output the author name in the body as a class like <body class="author-keanu"> or maybe they just use the author id like <body class="archive author-22"> and then you can check the author by going to www.example.com/?author=22 and maybe the template hierarchy has an author page, or a plugin added one, or maybe not.
Since there are lots of WP functions that can output/or use author/nicename or author id’s the only real solution is to audit your theme/plugin code.
Another important note is that you can guess if a username exists using WP’s login. for example if you guess an existing username of “Jane” the error will be:
ERROR: The password you entered for the username Jane is incorrect.