"I don’t believe thimerosal is used anymore. But autism rates still climb."
And the more conservatives parrot conspiracy theories that cannot win in a debate with actual research then the more they marginalize themselves. But it is reasonable to assume that "Conditions like autism are recognized by the Social Security Administration (SSA) as potentially disabling and may be able to qualify you or your child for Social Security Disability (SSD) benefits through one of both of the SSA’s disability programs" along with increased testing can have something to do with increased rates. https://www.scientificamerican.com/article/the-real-reasons-autism-rates-are-up-in-the-u-s/
Most of the studies I have seen find about 25% of the increase of autism cases can be attributed to better awareness and diagnosis. The other 75% is unknown.
The argument that those children would have been institutionalized doesn’t hold water either—there are too many autistic children per capita. If the rate were the same now as in the past there would not have been enough space in institutions for all of them. This doesn’t even consider other mental diseases.
People in the past were not stupid and autism would have been easily recognized by teachers, psychologists and the like. The disorder is in fact increasing and has been since the mid 1990’s.