Depends on the model, and even if you can get a custom rom on it, some of them will lose certain features with an unlocked bootloader. It’s not impossible to unlock them, but Samsung will kick and scream and break shit the whole way. That’s why it’s best to aim for a manufacturer that will allow bootloader unlocking. Pixel, Motorola, Sony, and I think most of the others will help you unlock the bootloader or at the very least won’t stop you. Samsung is easily the most spoiled child of the lot.