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Yes, since WWII things have been relatively peaceful, but the key term there is relatively. As we speak a pretty awful war is happening in Gaza, in the last twenty years we've had multiple wars with pretty severe casualties, and if you go a little farther back you get to things like Vietnam.
It's true that specifically atomic bombs haven't been used.
For the time being... Humanity's leaders are increasingly as insane (sometimes more-so) as their worshipers. I feel it's only a matter of time before atrocities and crimes against humanity skyrocket again. :(
Not even. We haven’t avoided nuclear war by not building nukes. And we still raze cities and manufacture incendiary weapons.
We still raze cities and drop incendiaries. America hasn’t gone to war with a near-peer nonnuclear power like Japan since WWII. To the extent we were faced with the prospect in the Cold War, both we and the Soviets were committed to MAD, i.e. using nukes. (Do you think unilateral disarmament in the Cold War would have lead to peace?)
There has been no militarily useful technology that was voluntarily abandoned. Just constrained. You can’t constrain a technology you don’t bother understanding.