I can see that topics are available in Chatter posts in Lightning Experience, but I can't see which topics are assigned to a given object when looking at its record.
For example, I might have assigned several topics to a given Account in classic by entering them in the banner across the top of the chatter section. I can then see those topics whenever I view the Account - they're visible in the same place as I entered them.
However, I can't see them anywhere in Lightning.