Growth on lilac bark
The lichen appearing on your lilacs suggests humidity and slow growth. An older lilac shrub growing slowly is natural and not necessarily a reason for alarm. The lichen is not corrupting the plant on its own. But while the lichen is not causing a problem, it may be a signal that your lilacs are in distress.
We have seen a lot of lichen appearing on trees in our area this summer as it was unusually moist in the early months. It also is possible that your tree is in too protected a position where moisture is accumulating and that any thinning of nearby plants that you can do to increase air flow will have a benefit.
I do see what appears to be newer growth on your plant but if you are seeing dying twigs or leaf loss out of the ordinary, then consider whether your lilacs are experiencing root problems from drought or excessive moisture. Lilacs are particularly unhappy to have lingering moisture around their roots. Adding compost to the soil to improve the texture and drainage would be wise if you see some signs of distress. Cut back the dying twigs and consider whether over-irrigation is a problem.