That is BS.
Congress could only pass SS to begin with as it promised that it would not be taxed.
There would never have been a law authorizing SS if that would have been the plan to begin with.
Go back to what you were originally seeking: Congress's proclivity to change the rules on taxing.
This is an example pure and simple.
You call it 'reconsidered'. I call it moving goal posts.
You can call it what you want it, but that doesn't change the facts.
And the mere fact that Congress could not have originally enacted social security if it had taxed the benefits is utterly irrelevant to whether, after sufficient experience with the system demonstrated that a significant number of people were getting an unwarranted windfall, a subsequent Congress could modify that system by taxing the benefits. It is a canard barely worth the restatement that one session of Congress cannot bind the actions of a subsequent session of Congress.