Android already published the full source code for Stable releases but barely anything for Beta releases. They didn't publish anything for upcoming releases with support for new devices. AOSP main branch received most changes through the Stable releases being merged into it. Most components were developed internally. Certain components were developed publicly in AOSP so they had to repeatedly merge back and forth between the internal and public main branches.
GrapheneOS would benefit from having early access to the monthly, quarterly and yearly releases as Android OEM partners do. The only benefit of having access to the main development branch would be the ability to backport more fixes than they do along with doing it earlier. We occasionally backported fixes from AOSP main for the few components developed publicly through it, which is what's going to be mostly going away. It was a big help to us as access to the upcoming quarterly and yearly releases would be. Monthly updates are too small for it to really matter but it'd still help.
Also worth noting the monthly security patch backports (Android Security Bulletins) are a separate thing from the new OS release each month. Those are backports of many of the High and Critical severity patches to older Android versions, often a month or two after they were released in the actual OS releases each month which are the monthly, quarterly and yearly releases (it's one of the 3 each month, with 3 quarterly releases and 1 yearly release per year although Android 16 is coming earlier this year instead of a 3rd quarterly release).