I believe the critical passage is here in CFR 14 §25.175¹ (2) With the landing gear retracted at low speed, the stick force curve must have a stable slope at a

I believe the critical passage is here in CFR 14 §25.175¹ (2) With the landing gear retracted at low speed, the stick force curve must have a stable slope at all speeds within a range which is the greater of 15 percent of the trim speed plus the resulting free return speed range, or 50 knots plus the resulting free return speed range, above and below the trim speed (except that the speed range need not include speeds less than 1.3 VSR1, nor speeds greater than the minimum speed of the applicable speed range prescribed in paragraph (b)(1), nor speeds that require a stick force of more than 50 pounds)[...] The crucial phrase is "stick force curve", which is the characteristic MCAS was created to tweak. As far as 'authoritative sources' go, I'll just mention the Gell-Mann amnesia effect.² (me: non-active aircraft dispatcher) ¹—https://www.ecfr.gov/cgi-bin/text-idx?node=14:1.0.1.3.11#se14.1.25_1175 https://www.ecfr.gov/cgi-bin/text-idx?node=14:1.0.1.3.11#se1... ²—https://en.wikipedia.org/wiki/Gell-Mann_amnesia_effect?wprov=sfla1 https://en.wikipedia.org/wiki/Gell-Mann_amnesia_effect?wprov...