There is something I miss from the Learjet 35 (one from FS95/98) that I don't see any more: Fuel tanks on the wingtips! What happened to the wingtip integrated fuel tanks in the business jet industry?
One of the main reasons wing tip tanks were used was to counteract some of the forces of wing bending by adding mass to the end of the wings. As time has progressed and our understanding of materials has grown as well as our ability to design structures better the need for tricks like this has gone away.
As our general understanding of aerodynamics has gotten better so has the understanding of the impact of wing tip shape on reducing drag. As such wing tip tanks have been traded out for wing tip devices.
In some cases the tip tanks are an option available as a later add-on or at purchase time. Generally this increases the range of the aircraft at the cost of useful load. As such, it may just be that most people who order planes these days prefer the higher useful load and the reduced range, rather than the extended range flying with a lighter load.
You can find some more discussion on it here.
More modern engines need less fuel for the same thrust, so the fuel volume needed for an acceptable range has shrunk over the last five decades. Wing volume alone is now more than sufficient.
While a typical 1950s era jet engine would consume 1 pound of fuel for every hour and pound of thrust (= 28 g/kN·s), comparable engines of recent design consume less than half of that (0.44 lb/lb-f·h in case of the PW500).
Note that even entry-level business jets have a range in excess of 2000 km, without tip tanks. The use of supercritical airfoils, which allow thicker wings for the same drag divergence Mach number, helped as well to make tip tanks superfluous.
The Lears with tip tanks were the 23-25 and the 35. They had laminar profiles with 9% wing thickness, the thin profile was to not have to apply wing sweep. The Lear 23 was developed from a Swiss fighter design, Bill Lear Sr. sold his electronics company, bought the design, and founded his bizjet corp. Hail Bill!
Internal fuel volume of a thin wing is a limiting factor. Storing fuel at the wing tip is a very good idea that is still carried out in all aircraft: internal fuel at the tip is used last as well. The external fuel tank further doubled as an aerodynamic barrier device, like a winglet.
However, tip tanks increase drag and therefore if at all possible to provide the range, will be stored inside the wing.