fatuheeva I'm wondering if it's possible to get a more specific timeline for end of support. I'm not sure why it needs to be so fuzzy.
At any moment somebody might discover a genuinely bad exploit in 4a firmware. As a completely hypothetical example, what if some day it turns out that a specially crafted SMS sent to a 4a could remotely wipe the flash storage? I don't know what the GrapheneOS project developers would do, but I suspect they would end legacy 4a support that same day.
For whatever reason, Google guaranteed firmware support only until August 2023. That time has passed. Every day of "support" after that is best-effort, as-available, not guaranteeable, etc.
Meanwhile, the project managers can roughly estimate the relative popularity of the 4a versus other devices by counting downloads. Over time, the 4a population will be smaller while the effort of building and testing GrapheneOS images will go up. At some point, not knowable now, those lines will cross.
Overall, today there is no way to say when it might be impossible to semi-suppport the 4a, and also today there is no clear way to estimate the costs and benefits N months out.
Please note that I do not speak for the GrapheneOS project.