If google is doing something as drastic as intervening in the installation of all apps, they're not likely to sell phones with unlocked bootloaders - the pixels that GrapheneOS currently depends on 100% - much longer.
Don't other vendors still sell unlockable phones? I first encountered Motorola back when they were assholes (pre Google digestion) but I thought their new devices were easily unlockable (if you vs carriers own them). Has that changed? My wife had a Motorola previously (she's went Apple recently and hates it) but Motorola post-Google seemed pretty nice.
a fairly large number can be unlocked, yes. Google's devices have just generally been the most visible because they've always been easy to target, and they have the biggest possible name behind them.