Google updated documentation for their Google Read Aloud user agent with more detail on how it works:

• It uses stateless rendering
• It needs to access a web page to see meta tags used on the page

Here's why it matters:

This user agent is triggered by a real user request so it ignores robots.txt rules - if for some reason you want to block it, you have to use the `` tag - but it's better not to block it because of accessibility and for users who just want to listen instead of read.

Stateless rendering means this user agent doesn't see cookies or logins, so if a site has a paywall and they don't want the paywalled content to be read aloud for free, then they have to use this structured data: `isAccessibleForFree` set to `False`