This is a new addition for 2020, though I'm sure there have been rules like this before.
In all classes, if it becomes apparent that one type or model of motorcycle gains an unfair performance advantage, AMA Pro Racing reserves at any time the right to implement or remove allowances or restrictions to restore competitive balance. Any allowance or restriction will be communicated via Competition Bulletin.
If a bike is legal at the start of the season, it should be legal all season. If the rules need changing, change them at the end of the season. There is a long precedent of this. I remember the Joe Kopp winning at the 2010 Arizona Mile on the Lloyd Brothers' Ducati and the bike being penalised with intake restrictors for the rest of the season. Kopp swapped to an XR750 and won another two races.
So what is an unfair advantage? Look at the Indian FTR750's record:
2019: Won 16 of the 18 Twins races
2018: Won 17 of the 18 Twins races
2017: Won 14 of the 18 Twins races
That means, since the Indian Wrecking Crew joined the series full-time for the start of the 2017 season, the FTR750 has won 47 of a possible 54 races. That is an 87% success rate. Is that an unfair performance advantage or a result of the best riders on the best bike? Or both. This isn't an anti-Indian post. Their bike is legal, homologated by AMA/AFT. Their three seasons of success follow years (decades) of dominance by Harley-Davidson. And if 75% of the grid are on the bike with the unfair advantage, is that unfair?
Or is the rule simply a catch-all for unforeseen tech that isn't covered by the rulebook?
Whatever there is the chance of a scenario a fellow fan pointed out to me: what happens to the rider who is in the form of their life and on their way to a title until the rules are changed on them?