Can a radiologist, after the taking of an mri of the leg, notice on the mri there was a venous closure (for varicose veins) procedure done yrs. ago?

Maybe: May see absence of vein, or chronic thrombus of saphenous, or if a device was placed that may image. Unlikely to see a stitch etc.

Venous sono.: The best test to show that a vein has been closed by a laser or radio frequency closure procedure would be a venous ultrasound. This is considered the gold standard for evaluating the results of closure procedures. Although an MRI may show the saphenous veins and whether or not they are patent, this is a very expensive test for those results.

Maybe: An ultrasound is a much better test to see this.

Difficult: By venous closure, I am assuming you mean venous ablation? It can be very difficult to determine that on an MRI, the test of choice would be ultrasound.