WP_Query() with custom post type and taxonomy - get all terms?

I would rather use a full proper tax_query here. The {tax} syntax is depreciated according to the docs

A tax_query is also better here as you will be dealing with an array of terms. When your URL is

http://www.myWebsiteURL.com/lessons

this means that all terms should be displayed. So this means that you need to use the full $validCats array as query terms.

With this in mind, I have slightly changed your code to the following: (CAVEAT: Untested, and I assume you are using term slugs here, so I have set the field parameter accordingly)

<?php
    /* if URL query value is a valid category, get all lessons for that category.
       If it is not a valid category (or not category is provided), get ALL lessons. */

    $validCats = ['study-skills', 'time-management', 'math', 'tutoring', 'reading', 'online-learning'];

    if ( isset( $_GET['lc'] ) && in_array( $_GET['lc'], $validCats ) ) {
        $terms = $_GET['lc'];
    } else {
        $terms = $validCats;
    }

    $paged = ( get_query_var( 'paged' ) ) ? get_query_var( 'paged' ) : 1;
    $args = [
        'post_type'   => 'lesson', 
        'post_status' => 'publish',
        'paged'       => $paged,
        'tax_query'   => [
            [
                'taxonomy' => 'subject',
                'field'    => 'slug',
                'terms'    => $terms
            ]
        ]
    ];

    $lessons = new WP_Query($args);
?>

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