To paste keyframes in reverse in Spine, you can manually achieve this by copying the keyframes you want to reverse, moving them to the right, and then dragging a selection box around them. Once selected, drag the left side of the box to the right side to invert the order of the keyframes. This effectively reverses the animation. While there isn't a direct "paste in reverse" feature, this method allows you to reverse keyframes manually.

Adelmo83 Thanks for your input! Spinebot's answer is correct, but we are actually aware that there should be another way to easily reverse the order of the keys, since the box selection method does not work in some cases, such as keys on fractional frames. Here is an issue ticket for this improvement, and I have added a link to this thread as a comment:
EsotericSoftware/spine-editor819
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