A recent episode (season 6, episode 11) of Young Sheldon predicted Leonard's role in The Big Bang Theory. Dr. Sturgis offered advice to Sheldon about roommates becoming lifelong friends and this perfectly described Leonard's relationship with Sheldon in The Big Bang Theory.
Although the sitcom focused on Sheldon's relationship with Amy during the finale, it is important to remember how pivotal Leonard was in Sheldon's life, especially after becoming his roommate. In the prequel series, Young Sheldon, Leonard's absence is highlighted when Sheldon is left to his own devices, making their bond even more significant. This insight from Dr. Sturgis shows just how important Leonard was to Sheldon's development as a character in both shows.
