the influencer marketing is quite big but probably harder for casinos and gambling companies to announce too
not sure about twitch and other networks, but as far as I remember youtube has some regulations to make it forbidden.
There are some rules to regulate it, but it is not forbidden. Becoming an influencer or sponsored player is probably also a dream of many players, but the market seems already oversaturated right now. You got a lot of popular streamers that get more than 100k views on every video they upload on youtube.