20' run of romex unsupported through confined-space basement finished ceiling?
This is fine
What you are describing falls under NEC 334.30(B) point 1:

(B) Unsupported Cables. Nonmetallic-sheathed cable shall be
permitted to be unsupported where the cable:
(1) Is fished between access points through concealed spaces
in finished buildings or structures and supporting is
impracticable.

So, go ahead and fish the cable through the ceiling as you plan to do.