The antonyms for the phrase "climbed onto" can vary depending on the context. In some cases, antonyms might include "descended from," "dismounted from," or "stepped down from." These antonyms suggest that the subject has moved from a higher position to a lower one, as opposed to "climbing onto" something. However, in other contexts, the opposite of "climbed onto" might not involve the concept of height at all. For example, in the context of a debate or discussion, the opposite might be "stepped back" or "withdrew from," indicating that the subject is not engaging as actively as before. Ultimately, the antonyms for "climbed onto" will depend on the specific context and the word's intended meaning.