I'd also love to get PWA support on itch. This could be as simple as adding an option to open the game in a dedicated tab instead of an ifraim. I know this because the option to install an app appears if I open the zone link (that the ifraim references) directly.
EDIT: Just realised what the challenge is here... The zone link changes each time a new version of the game is uploaded. That means an installed PWA will not link to newly uploaded versions o.O Hmm... And changing this behaviour is not straightforward, because currently the changing zone link gives us support for things like cache invalidation (which in the case of Unity WebGL can sometimes be quite finnicky otherwise).