Are user-created leagues deleted if they fall to fewer than 16 members?

i would say NO

Talk is Cheap League = https://billsmafia.myfootballnow.com/

this league has 20 openings yet it doesnt have the date it will expire like Custom 10 , Custom 12 , Custom 17 and Custom 77 do

Custom 10 = https://cust10.myfootballnow.com/ 4-10-2018
Custom 12 = https://cust12.myfootballnow.com/ 4-05-2018
Custom 17 = https://cust17.myfootballnow.com/ 4-14-2018
Custom 77 = https://cust77.myfootballnow.com/ 4-13-2018

No, they will not automatically delete like the standard leagues will, because someone did pay to create them.

Then none should delete.