I'd love to see mushroom cultivation, that could lead into some proper cave farming for once; Imagine using the rot you collect from your spoiled food to cultivate new food in the form of mushrooms, it'd form a new way to process waste material like that. And mushroom cultivation appears to have been around since the BC's in china so its def not out of the scope of the game.
Though I have to mention some oddities to your suggestion, in game currently all the mushrooms spawn on grass which poses a problem, how is it going to interact with that?
Will the mycelium exist under the grass? Are you expecting a mycelium grass block?
as for digging it up I'm not sure the holding shift method is good, I feel there's a more elegant solution, maybe the player could hold right click to harvest it with the right tool like berry bushes and resin from pine trees which would turn the mycelium back into a dirt block and also give you a chance to get an item that lets you create a new mycelium block.
How big would you expect general mycelium networks to be as well, if a player digs down I doubt they should be swarmed with mycelium despite the fact that in forests it is everywhere so its not that simple of a suggestion to get right but it is one I like.