[deleted] I'm just a moderator, not a decision maker for the project, so don't take what I say as a final word, but I'm pretty sure I recall something like this has been requested/discussed before.
The issue is just removing the logo won't do much if someone who knows what they're doing really takes a look at the phone. Before booting, a hash is displayed and it's not hard to determine the OS is GrapheneOS. If they get into recovery, the name "GrapheneOS" shows up instead of generic "Android" (I haven't checked, but saw it was rebranded on GitHub, so I think I should be right about this part). If someone either gets the phone while it's unlocked, or can unlock it somehow, figuring out the OS is very simple because of preinstalled apps, features, etc.
I don't know this for sure, but I think it probably isn't possible to give users the option to change the boot logo in AOSP. Maybe GrapheneOS developers could add that functionality, but considering someone with access to the phone can figure it out anyway, I'm unsure what benefit it would have.