While transiting through Mexico City today, I received a receipt that looked like this. Unfortunately, I didn't photograph mine, so this is a photo I found online, but it looked exactly the same. The receipt states:
Welcome to Mexico
Important and Mandatory Notice
Save your ticket. In order to depart Mexico, you MUST scan the following QR code, or enter the URL https://www.inm.gob.mx/spublic/portal/inmex to obtain your Digital Multiple Migratory Form. You MUST download it and have it ready to exit Mexico. Please take note: if you do not comply, you will not be able to leave the country. Thank you for your compliance and thank you for visiting Mexico. Keep this ticket until you leave Mexico. It will be required.
I only noticed the MUST download part while I was standing in line for departure, but the gate agent just took my receipt and didn't ask to see anything else. So is it actually required for anything in practice?