To answer your question - why answer this after 5 years?
Point 1: The question after 5 years was not answered.
The question: I was wondering on why you would even have to report SSA-1099 at all?
Google Gemini [GG] gives advice:
[GG] Key Takeaway: Even if your Social Security benefits are not taxable in the US due to the treaty, reporting the SSA-1099 and filing Form 8833 is crucial to demonstrate your compliance with US tax laws and claim the treaty benefits correctly.
Point 2: TurboTax emailed me the following - causing me to re-evaluate my assumptions about the taxation of social security benefits in the US versus contries where there is an agreement (see list in THIS thread):
[Turbotax] We’re contacting you because there’s an issue with the accuracy of your 2023 tax return. Due to a TurboTax error, some customers might have overpaid their federal, and if applicable, state taxes.
From reading through this thread, it appears clear that for YEARS people have been HACKING their US federal tax returns in an attempt to zero out the taxes from reporting the SSA-1099 income on which there has been no tax withholding due to the agreement. As of this moment, I am trying to figure out if there is an official "HACK" that turbotax NOW EMPLOYS to zero out the US federal taxes. Stay tuned.
Notes: Edited for clarity/typos and added a link to a different relevant thread.
