The shoe insert has actually been built, though I haven't heard if it, or something similar, has been used in real cheating yet.

It's a shoe-to-Pi device called Sockfish (a pun on the Stockfish chess engine): https://incoherency.co.uk/blog/stories/sockfish.html

You don't even have to relay the move to the player, simply giving a sign that this move is important can be enough for the player to understand they've got the edge and utilize that. It could be as simple as someone in view giving a sign with body language. Remember also that a professional chess player doesn't need to cheat every single move, cheating once in a game at the right move can be enough to put the game in your favor at such a high level.

Wow, this goes deep!

Well done!

And all of those are extremely easy to prevent/spot in a tournament setting. Carlsen is just throwing a tantrum here, IMO. Sore loser.