Bumping this - Messages for Web should be in the same category as Whatsapp, FB Messenger, Telegram, etc. Slapping it into the sidebar would certainly be a massive value add for Android users (especially as this isn't a feature in Chrome - this establishes Opera as a better and more congruous cross-platform experience).
I guess this is more of a generalised CX point but this could also be resolved by allowing users to add their own "messengers" by plugging in additional URLs into that setup section. Means fewer of these forum posts and that users can update their own browser quickly if a rogue CEO decides to change the name and URL of a platform on a whim 