Great work. One error that I spotted two hours ago was actually corrected before I typed this. Just some thoughts, questions and minor errata:
A. "Community Day Venusaur is debatably the best Grass attacker now in GO"? Not any longer. (In fact, I would demote it a tier, despite Bulbasaur being my favourite starter.)
B. "For the people that have it, EX Mewtwo is strictly better" than Alakazam? Psychic was not made legacy during Mewtwo month.
C. The defensive rebalance gave Gengar more bulk, thus its promotion from Tier 4 to 1.5, but the effectiveness rebalance undid that. Gengar and Rampardos do not deserve to be in the same tier as Tyranitar, which has two great movesets (albeit one legacy), is also decent on gym offence, an absolute monster in Master League PvP and far more accessible.
D. Does Moltres still deserve to be two tiers higher than Entei? Its promotion was back when we expected Heatran to outclass both, but Heatran turned out to be a flop. The two types where Sky Attack has an advantage are less relevant than the two where it is useless (Metagross is a very common gym defender).
E. Consider placing Gardevoir in Tier 4. The HP buff made Machamp solos no longer a joke and her bulk can help avoid wiping out against Dynamic Punch (which is hard to dodge often weather-boosted). I regularly use her on gym offence against Dragon-types and Fighting-types (of course, Mamoswine is coming). Palkia is not the first legendary dragon where I used her to anchor my team (I also did so for the Lati twins). Of course, Mamoswine is coming...