We have had a large number of questions like yours on lilac this year. Conditions have been ideal( humid, wet, warm) for leaf spot diseases on lilac. It is too late to apply anything to the plant, other than keeping it watered during dry times. Redirect sprinklers, or water early in the day, so leaves are kept dry or will be dried quickly in the sun.

I can't be sure of what is causing the problems, but it does look similar to bacterial blight or psuedocercospora leaf spot. There are bulletins below on leaf diseases.

Rake up all fallen debris this fall to minimize overwintering pathogens.

https://extension.oregonstate.edu/news/how-recognize-treat-avoid-lilac-bacterial-blight

https://hnr.k-state.edu/extension/horticulture-resource-center/common-pest-problems/documents/Lilac_Pseudocercospora_Leaf_Spot.pdf

https://lancaster.unl.edu/hort/articles/2005/prunelilacs.shtml

Since this is affecting several lilacs, consider submitting samples to the MSU Plant and Pest Diagnostic lab. An exact identification will prepare you to apply the correct preventitive next spring. There is a modest fee for the service.

https://www.canr.msu.edu/pestid/

A 3 stage “rejuvenation pruning" on some of your largest lilacs may be helpful to get some new fresh growth.

https://lancaster.unl.edu/rejuvenating-older-lilacs