In case someone bumps here again, I found two ways it can be achieved within a theme without using any using module.

1. Using custom CSS:

   .user-logged-in .menu.menu--account a[href="/user/register"] { display: none; }

2. Using custom JS:

   $('.user-logged-in').find('.menu.menu--account a[href="/user/register"]').hide();

This is applicable only if you are customizing your theme in any way.

In case someone bumps here again, I found two ways it can be achieved within a theme without using any using module.

1. Using custom CSS:

   .user-logged-in .menu.menu--account a[href="/user/register"] { display: none; }

2. Using custom JS:

   $('.user-logged-in').find('.menu.menu--account a[href="/user/register"]').hide();

This is applicable only if you are customizing your theme in any way.