I am unable to enlarge your photo so will have to guess.  It looks like you have some galls but I can't get a good enough look to be sure.  Galls are not harmful but a cosmetic problem. Mites lay eggs on the leaves and the leaves react with a growth, similar to when we get a scab on an injury.  The egg will hatch and the tiny insect goes on its way.  If there are insects feeding, we would need a good photo of them to identify.  More than likely they have fed and gone away.  You could try spraying some all season horticultural oil in the early spring to discourage insects.  Sorry I can't be of more help but your linden should be okay.  The result is most cosmetic.  https://extension.umn.edu/yard-and-garden-insects/insect-and-mite-galls