Should the fill line be removed when decommissioning an old oil tank?
Whether or not they remove the fill line, depends on how the tank is decommissioned.  There are three ways an abandoned tank can be handled, as explained in this PDF

The 1997 Uniform Fire Code, adapted by WAC 51- 44, requires that
heating oil tanks out of service  for a period of one year shall be
decommissioned  by using one of the following processes.

Removal from the ground and restoration of  the site in an approved manner.
Abandonment in place by filling the tank completely with an approved, inert solid material.
Tanks of 1,100 gallons or less may be left empty provided they are first pumped and cleaned,  and have the fill line capped or plugged,
below  grade, to prevent refilling of the tank.  (NOTE: Some local
juisdictions do not permit  this “clean and cap” method. Check local
requirements before beginning any decommissioning process.)


In the first method, the tank is completely removed, so the fill line will also be removed. With the second method, the fill line may be left behind.  In the third case, the fill line should be cut below grade before it's capped. So the fill line will still be there, but it will be at or below the level of the ground.
The document also has a good list of questions to ask the contractor.

Some important questions to ask contractors:
• Are they experienced?
Can they provide the  names and telephone numbers of current or
recent customers as references?
• Do they have environmental
pollution liability  coverage?
• Do they collect soil samples?
• Which laboratory do they use for the analyses?
• Where will the tank, oil, and contaminated
water from rinsing out the tank be disposed? Is
that disposal site insured?
• What documentation, labeling, and other
paperwork are provided? You should receive
documentation of disposal of the tank, a copy
of the lab results, documentation that the soil
samples were handled properly, a copy of any
permits required, and documentation of the
disposal and/or treatment of any wastes.