A lot of planes still use elliptical wings - sort of.
What the maths tell us is that the most efficient wing configuration for a given wing span should have an elliptical lift distribution*. The most obvious way to implement this is to make your wings elliptical.
But aircraft designers learned from the experience of manufacturing the Spitfire that elliptical wings are more difficult to manufacture leading to increased cost and manufacturing time.
So later in the war when resources became tight and everyone assumed that they were racing the Germans to build better, faster planes designers deliberately chose to use straight wings to ease production and reduce costs. The P51 Mustang was designed this way.
But we have learned that making your wing elliptical isn't the only way to have elliptical lift distribution. To get elliptical lift distribution you can:
- Make your wing more elliptical
- Add washout to tune lift distribution along the wing
- Change airfoil profile from root to tip to change lift distribution
- Do any combination of the above (eg. washout + rounded tips)
So a lot of planes still use elliptical wings. Especially when fuel economy is one of the main driving design objective. It's just that they don't look elliptical.
* note: There is evidence that this may not be accurate. It is true that if you fix your wingspan the maths will output an elliptical distribution but if you fix your weight (ie. lift at cruise) the most efficient distribution turns out to be something else (something bell shaped) but you end up needing to extend your wingspan