Is it possible to repair a Toshiba 50L3400U Smart TV?

It all depends, what has gone wrong and what needs fixing, but generally, yes.
The internals now have been reduced to very few modules. A power supply card, a video and computer card and an interface card. Most repairs are done by replacing a complete module rather than fixing a board fault. (Although if you can find a good technician, this is still possible.)
Unfortunately the internals are made by another company (Like Vestel) rather than Toshiba, and these boards are fitted to many brands of TV.
There are minor differences on cards of the same part number and the changes even happen with the same model of TV. Screens are often from several different suppliers, with differing connection cables.
It is a mine field, trying to track down the right components. Toshiba are not interested, outside of their legal reqirements of supplying goods suitable for purpose. They would rather sell you a new TV.