It is critical to understand that you should not use 在下 in normal, everyday conversation. Doing so would sound strange, overly theatrical, or even sarcastic. Its modern use is highly contextual.
Historical Media (Wuxia/Guxia): This is the most common place you will hear 在下 today. It is the standard pronoun for swordsmen, scholars, officials, and anyone trying to be polite in a historical setting. When a wandering hero introduces himself in a film, he will almost certainly use 在下.
Online and Gaming Culture: In online forums, social media, or games with historical or fantasy themes, users sometimes adopt personas. Someone might use 在下 humorously to role-play as a character from the “Jianghu” (江湖, the martial arts world), adding a touch of chivalrous flair to their posts.
Formal and Literary Writing: In very formal or artistic writing, an author might use 在下 to evoke a classical, elegant, or historical tone. This is a stylistic choice and is quite rare.