Are US Social Security recipients actively being targeted by ACRA to pay tax on that income? Probably not.
Is that income taxable in Argentina? It will depend on personal circumstances. The answer is ‘yes’ if the person meets the conditions for tax residency under articles 116 to 123 of the Ley de Ganancias (Law 20,628, (2019 Texto Ordenado)) AND the social security income is above the minimum threshold (which changes over time and depends on a range of personal circumstances). The thresholds are currently quite generous (the USD equivalent of roughly 2,900,000 pesos per month for a single person, but higher for someone with dependents), meaning few people (even foreigners with overseas pensions) have much, if anything, to pay. Furthermore, there is a special discount for pensioners (deducción especial para jubilados y pensionados) arising from the 2019 Garcia case in the Supreme Court, which further lowers the threshold, provided US Social Security meets the conditions set out in Article 30 (of the same law). US Social Security may well be an income type that triggers the deducción especial, but I’ve not looked into it, since my pension is from another country. Also, if there IS any tax to pay, any tax already paid in the US could probably be used as a tax credit in Argentina.
In summary: It is a complex area, and each person who wishes to be compliant needs to obtain specialist advice. Unless someone has quite high social security monthly income, they may have nothing to declare (under current thresholds) even having met the conditions for tax residency.
(There is certainly no exemption under Argentine law for US Social Security income. Also, regardless of thresholds for income (ganancias) taxation, any person who has met the conditions for tax residency under articles 116 to 123 simultaneously becomes liable for the bienes personales (wealth) tax on assets held around the world under Article 30 of Law 27.541. Which is a whole other story.)
(Anyone interested can do a Google search on these two laws and the articles cited to see for themselves; I don't copy them here because that would make for a very long post.)