Android has always been developed in a closed-source manner by Google engineers, the recent changes only reduces the visibility of ongoing changes and the ability for developers outside of OEMs to contribute to Android (such contributions were already rare).

This is explained further in this article:

While some OS components, such as Android’s Bluetooth stack, are developed publicly in the AOSP branch, most components, including the core Android OS framework, are developed privately within Google’s internal branch. Google confirmed to Android Authority that it will soon shift all Android OS development to its internal branch, a change intended to streamline its development process.

Do it! Do it! Do it!