With the move to subplugins.json @ MDL-65646 initially we kept the original subplugins.php files with a debugging call aiming to warn to any non-core access to those files.

But those early debugging calls were, after all, detected to be not really useful, possibly redundant (core_component also has own debugging implemented for plugins missing the json files) and prone to errors (they are loaded really early in the bootstrap/warmup of Moodle and debugging() facilities are not available yet.

So @ MDL-65995 it was reconsidered if we should keep them and, ultimately agreed that there wasn't much real use case for them.

So this issue is about to remove all the (now) old subplugins.php files in core. Once and forever.

Ciao