If you inspect the webpage from Chrome/Firefox devtools and check the iframe which loads the HTML5 game you see the allow option on the tag.
On the contrary. It could potentially open up for a new revenue stream for itch.io. They could set their own payment pointer if the game doesn’t have one for instance. There’s also an opportunity for itch.io to apply for a grant from Grant for the Web to get funding for this.
Getting platform providers and game portals on board is very important for adoption of web monetization as a standard.
There are discussions with Gamejolt regarding this. Not sure about Indiexpo.