Tax Cuts and Jobs Act of 2017 moved domestic R&E expenditures (including salaries) under IRS Sec 174 from a same-year credit to a five-year amortized expense (similar to capital expenditures). It also amended 174(c)(3) to ensure that software dev is unequivocally an R&E expense[^1].
1: https://irc.bloombergtax.com/public/uscode/doc/irc/section_1...