Not the ADA specifically, but American courts have generally held that large commercial websites do need to be accessible in terms of the ADA.
That's aside from the cruelty of diminishing accessibility on a major social platform, slowly cutting off disabled users, despite its cost likely being a rounding error in the greater scheme of things.
"In the United States alone, approximately 50 million people are considered deaf or hard of hearing. The failure to provide appropriate accommodations to this community is a violation of the Americans with Disabilities Act and the Rehabilitation Act of 1973."
If you read through the case history, you can see that the ADA does not have specific language saying something like "websites above size x must be accessible, and that means a, b, and c need to be followed." Much of that has been developed into case law and precedent by judges ruling in numerous lawsuits over the years, which is why it's still a somewhat grey area for edge cases.
That's why I said diminishing accessibility and slowly cutting off disabled users. This obviously isn't going to happen immediately, but every new feature will be less accessible and maybe even not provide accommodations entirely.
Possibly true but I would imagine the team created an internal Bible of all accessibility requirements and that all of that information wasn't siloed in the heads of each employee in that department.
