My understanding is that you will fill out a 8833 if you are claiming a treaty exemption for Social Security income in accordance with this Turbo Tax post written by Irene2805. You are correct that this shouldn't be reported as US Social Security income but reported as Irene 2805 suggests. I would report this as other misc income with the offsetting negative value. UK Social Security can be claimed as a treaty exemption and is not taxable if you claim a treaty-based position with a proper 8833 filed with the return.
As far as why Turbo Tax does not support 8833, we don't have a specific answer. It could be programming, complex calculations that aren't supported. This is information that we have no access to thus we are unable comment on.
As an FYI, these forms are not supported by Turbo Tax. It does suggest in this link that the reason why they are not reported stems from the fact that few taxpayers actually need these forms.
@theTA007
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