Fixing Deprecated: get_the_author_lastname

by

Note: I’m not simply giving you the full fixed code, but the line in which you need to make the changes is the one with the <span class="Z3988" ...>.

So yes, both get_the_author_firstname() and get_the_author_lastname() have been deprecated and as said in the error, you should instead be using get_the_author_meta().

And in the coinsify_the_content() function, just find and:

  1. Replace the get_the_author_firstname() with get_the_author_meta( 'first_name' ).

  2. Replace the get_the_author_lastname() with get_the_author_meta( 'last_name' ).