Just a quick followup: in the 2020 NEC, what was 310.15(B)(7) has been moved to 310.12. As such it is clearly no longer subject to the "as modified" language in 310.15, and simply says you are allowed under the specified circumstances to use a conductor whose ampacity is smaller than the service/feeder rating.
Depending on your point of view, you could consider this a change from 2017 to 2020, or a clarification on the intention of the 2017 NEC.
Cheers,
Wayne
P.S. The 2023 first draft report recognizes that 240.4 needs an exception for 310.12, which allows the use of conductors of lower ampacity than the service/feeder rating. So it now has a new section 240.4(H). One could argue under the 2020 NEC that 310.12 is moot, as it has no such allowance in 240.4.