Many have unlockable bootloaders (though the number is rapidly declining with Samsung closing up). But not many have relockable bootloaders. This is one of the things that grapheneos have set as a minimum standard, hence the reliance on pixels. There's a few other specific things that the titan chip provides which they rely on but the relocking is the main thing.

To be more specific - relockable with a custom AVB key. I think most devices can relock with the default Google AVB key.