How is it that entering the lavatory with the left foot and exiting with the right foot is from the guidance of the Prophet (peace and blessings be upon him), while there were no bathrooms in his era?

The jurists have agreed upon the desirability of advancing the left foot upon entering the lavatory and the right foot upon exiting it. There is no specific evidence from the Sunnah for this matter, but scholars have inferred it based on two lines of reasoning: 1. The Juristic Principle: Advancing the right (foot/side) in everything that signifies honor and distinction, and advancing the left (foot/side) in everything that pertains to harm or its opposite. 2. Analogy to Entering the Mosque: The right foot is advanced when entering the mosque and the left foot when exiting it. The lavatory is the opposite of that, so the left foot is advanced upon entering it and the right foot upon exiting it.

This etiquette is not limited to entering a built lavatory; it also includes relieving oneself in open spaces. In such cases, one advances the left foot when reaching the place of sitting and the right foot when departing.