Handshakes can form legal contracts, and contracts can be formed orally.
While true, these are terrible forms of contract agreement for anything of value, and specifically when there are no witnesses. One person could easily claim that "I heard them say something else" or "We didn't shake on anything!".
As for emojis, you can interpret them in 101 ways, and that's assuming both parties are using the same emoji icon set! Some look different depending on the platform, and some have completely different meanings without even knowing it! When I get an emoji on my business email, it doesn't even show up as an emoji!
A "thumbs up", in my book, is not an agreement to a contract. I want a clear written acknowledgement and/or a signature. Anything less could be hard to prove or completely denied as even happening.