Many 3d games use 2d sprites. A common term for a 2d sprite which always faces the camera in a 3d environment is a "Billboard". It can be a neat trick to bring detailed objects into a scene while keeping the polygon count low. A complex object which would otherwise take hundreds or even thousands of polygons can be reduced to just two when you replace it with a pre-rendered billboard. A prominent example are the trees in the intro of Super Mario 64. Notice how they don't change when the camera moves up or sideways.
But more modern games also occasionally use this technique. It is especially popular if you need to render a large number of detailed objects which are far away from the camera. In that case you just render the object once to a texture and then use that texture as a billboard instead of the actual object. This looks bad if you do it with objects which are very close to the camera. But when the objects are very far away, it can be almost impossible to see. Such a sprite is called an "Imposter".
And then there are games which combine 2d and 3d elements as an aesthetic choice. One well-known example is the Disgaea series which uses 2d character sprites in a 3d environment. The sprites got more detailed over the course of 14 years with 6 games, but they never gave in to the temptation to go fully 3d:
Short Answer: Yes, there can be 2D sprites/ assets in a 3D game.
I am just gonna explain you the general idea and what most game-engines usually use. What is 3D, it's 3-dimensional which means it automatically encloses 2D.
In fact, most game-engines these days allow you to work freely between 2D and 3D (Unity for example). The 2D mode in Unity is simply 3D with 2D perspective.
This is the list of a few games that mix 2D and 3D: