Learn Spanish (Spain): Desgraciadamente, todavía no ha llegado el jefe. - Unfortunately, the boss still has not arrived.

Not necessarily, but it is the most natural position for a sentence adverb. Words like desgraciadamente (unfortunately) or francamente (frankly) don't describe how an action is performed; they express the speaker's attitude toward the whole situation. Putting it first sets the emotional tone immediately.

Is the comma after Desgraciadamente required?

Yes. Because it modifies the entire sentence rather than a specific verb or adjective, it needs to be isolated with a comma. If you moved it to the end of the sentence, it would still require a comma before it: Todavía no ha llegado el jefe, desgraciadamente.

Why is the subject (el jefe) at the very end of the sentence?

Spanish word order is quite flexible. With intransitive verbs (verbs that don't take a direct object) related to movement, arrival, or appearance—like llegar, venir, or aparecer—it is completely normal and often preferred to put the subject after the verb. El jefe todavía no ha llegado is also grammatically correct, but putting el jefe at the end flows very naturally in spoken Spanish.

In Spain, why use the present perfect (ha llegado) here instead of the preterite (llegó)?