There’s a gray bar across the top that says, “This site is currently in read-only mode; we’ll return with full functionality soon.”
That’s not a hidden feature of Always On Availability Groups. Rather, it’s a hidden feature of really dedicated developers whose application:
Tries to connect to the SQL Server, and if that fails, shows a total-site-down page
If it’s not writeable, shows a polite banner across the top of the site, and still gracefully runs readable queries only
This is where a bit of foresight and hard work can really pay off. Read the whole thing.