Today it was announced that in total 34 events have been cancelled, due to lack of participation of at least 5 NOC's. This also means that Water Ski has been removed from the program.
In many of those, Mexico and the hosts had good prospects to medal, and hasn't been uncommon in the past two editions that other countries (other than Mexico) have removed or not entered their athletes on purpose to hurt their main rivals chances.
Anyway, here's the list of the events that were removed, althoughI'm missing one of the events, but I couldn't find it anywhere.
Aquatics - Diving (2) Women's 3m Synchronized Springboard & Women's 10m Synchronized Platform.
Athletics (5): Men's Marathon, Men's 50km Race Walk, Women's 10,000m, Women's 3,000m Steeplechase & Women's 20km Race Walk
Canoeing (3): Women's K4 500m, Women's C1 200m & Women's C2 500m
Cycling - Track (1): Women's Team Sprint
Gymnastics- Rhythmic (1): Team Event
Gymnastics- Trampoline (1): Women's Synchronized
Modern Pentathlon (2): Women's Relay & Mixed Relay
Roller Sports - Figure Inline Skating (1): Women's Individual Dance
Rowing (1): Women's Pair
Sailing (2): Women's Windsurf & Mixed Lightning Class
Shooting (1): Mixed Trap
Taekwondo - Poomsae (1): Mixed Pairs
Water Skiing (6): Men's Figures, Men's Slalom, Men's Wakeboard, Women's Figures, Women's Slalom & Women's Wakeboard
Weightlifting (6): Men's 77kg Snatch, Men's 77kg Clean & Jerk, Women's 90kg Snatch, Women's 90 Clean & Jerk, Women's +90kg Snatch & Women's +90kg Clean & Jerk.