Damage to lilac bush.
Came back from travel in July. Thought it might be from heat and lack of rain but it continues to lose leaves but regrow. Damage seems to be from happening from the bottom up. I have seen a stink bug on the stems occasionally... Thanks. Beth
Lilacs are susceptible to a bacterial disease, which may be a factor in some of the symptoms you are seeing. https://extension.umd.edu/hgic/topics/bacterial-blights-lilac For the most part, however, it looks like your plants are showing symptoms from stress, potentially from drought and very high temperatures. Many landscape plants are now showing end-of-season symptoms and decline, but by this time in the growing season, plants have stored enough food reserves to be able to generate new growth next spring.
The gray-green patches on the stems in your third photo are lichens. They are not harmful to landscape plants. https://extension.umd.edu/hgic/topics/lichens
Lilacs benefit from regular rejuvenation pruning to stimulate the growth of new shoots. You can cut the oldest and weakest stems to the ground in the winter. Beyond pruning, there is nothing we can recommend at this point to clear up the declining foliage; it is finished for the season. You have healthy buds on some of the stems shown in your first photo. Keep your plants watered if your area has periods of drought.