This originated from #2408 / #2408 (comment).

The AMP plugin currently requires WP 4.9+ and PHP 5.4+ for a few reasons:

• The plugin should be able to run on as many sites as possible.
• 4.9 is the last version without Gutenberg support, so it was assumed that transition period is a bit longer
• There were no drastic benefits in updating the WordPress version requirements
• There were no drastic benefits in updating the PHP version requirements

That means we currently need to support the following scenarios:

• WP 4.9
• WP 4.9 with Gutenberg plugin 4.9
• WP 5.0+ (Gutenberg built-in)
• WP 5.0+ with Gutenberg plugin

Gutenberg will eventually drop support for PHP 5.2 and require WordPress 5.2+ (which requires PHP 5.6):

• Remove support for PHP 5.2 and 5.3 WordPress/gutenberg#15796
• Increase WordPress minimum to 5.2.0 WordPress/gutenberg#15809

That makes everything a bit more complex.

In related news, Yoast SEO is going back to their policy of only supporting WordPress latest and latest - 1: https://yoast.com/supporting-older-versions-of-wordpress/

One possibility to reduce our workload would be to bump requirements for PHP and WordPress as well. Perhaps with the AMP plugin 2.0 we could move to PHP 5.6 and WordPress latest and latest - 1 as well.