I've probably decrypted ~200 Decoherent Engrams since the patch and gotten... I think 2 Legendary items? I only remember The Swarm with Field Scout I got a few days ago. The odds seem to be around 1%.
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That seems right I was thinking between 1-2%
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I have over 100 with NO legendaries... It's all anecdotal, but much less than 1%.