It's not much power, so it's not likely to cause major or permanent damage, but it may affect others differently and could cause burns if left on too long like if someone is less sensitive and doesn't think it's live.

And if the person is grounded and if they touch the hot side of the battery first there's a chance the charge could travel through the body rather than just the tongue. It's not enough to affect a heart, but might disrupt a pacemaker or other embedded device.

And of the battery is leaking, it could cause permanent damage from chemical burns from the alkaline and poisoning from heavy metals which while unlikely to be deadly with just one battery, heavy metal poisoning is cumulative across a lifetime.

So under ideal circumstances it is safe, but there are always risks with electricity and toxic chemicals, though relatively small.