Under current and future laws, Social Security benefits are subject to federal income taxes above certain levels of combined income (see table below). Combined income generally consists of your adjusted gross income (AGI), nontaxable interest, and one-half of your Social Security benefits.
What has changed are the applicable tax brackets—the new law lowered most tax rates and adjusted the income thresholds for the different tax brackets (get details). So the taxes paid on the same Social Security benefit could be lower.