Delta is less deadly because more people have some immunity, either because of vaccination or because they have already been infected. Also it came late in the game, and many of the people who are the most at risk are already dead. Treatment has improved too.
So the virus itself isn't less deadly, it is just that we are better prepared.
the relative deadliness of delta... we'll just not ever really know. Though IIRC, some early surveillance with all the controls (only looking at unvaccinated, etc) suggested that it was LESS deadly for the elderly, more deadly for other age groups, net less deadly.
So the virus itself isn't less deadly, it is just that we are better prepared.