I'm planning an itinerary where we would drive from San Diego to Tijuana airport (or possibly cross on foot), then fly to South America. It appears that paper FMMs are now being phased out and no longer issued at Mexican airports.
But... how is the FMM rule now enforced if leaving by air? Are you supposed to go show your paper FMM to someone at Tijuana airport? Earlier this year I've left Mexico via Puerto Vallarta airport and AFAIK nobody looked at my Mexican immigration stamp, nor were there any signs about FMM checks.