How to tell if a Google App Engine documentation page applies to the 1st/2nd generation standard or the flexible environment

I see a lot of google-app-engine questions from users mixing up the standard environment with the flexible environment, especially when it comes to documentation, often attempting to follow the wrong instructions, obviously with undesired results.

How can one easily tell which GAE environment a certain page is applicable to?

Most (if not all) of the GAE documentation is structured by the environment. When looking at a certain documentation page the environment is clearly indicated in several places:

• the second section at the top of the left-side documentation navigation bar
• the navigation trail at the top of the actual documentation page (which may vary slightly depending on the programming language)
• the docs/flexible vs docs/standard strings in the doc page URLs
• some pages applicable to multiple programming languages and/or environments (maybe with small differences) have tabs indicating:
  • the currently selected language and environment
  • the other languages and environments the page applies to (selectable)
  • the languages and environments the page doesn't apply to (grayed out)

Examples:

If the documentation page doesn't show the above mentioned patterns (for example if it's a page about some other Google Cloud product somehow related to the App Engine), then check about the App Engine feature being discussed and try to locate the respective GAE documentation.

What may also help with the distinction would be to check these guides putting the 2 environments and/or their features side by side:

• Choosing an App Engine Environment
• App Engine Flexible Environment for Users of App Engine Standard Environment
• Migrating Services from the Standard Environment to the Flexible Environment

Note: when multiple language versions are listed/mentioned under the standard environment (excepting Node.js) they refer to the 1st and 2nd generation standard environment, significantly different from each-other. You should also check the comparison and/or migration guides for these cases:

• Understanding differences between Python 2 and Python 3 on the App Engine standard environment
• Migrating from Java 7 to Java 8 Runtime
• Migrating Your App from PHP 5.5 to PHP 7.2
• Migrating your App Engine app from Go 1.9 to Go 1.11