While it is difficult to accurately diagnose from the photos, it appears that your lilac might be suffering from Pseudocercospora fungus.  As noted in this article, it is not likely to kill the plant unless infected multiple years in a row:

https://extension.missouri.edu/media/wysiwyg/Extensiondata/Pro/PlantDiagnosticClinic/Docs/leaf-spot-on-lilac.pdf

Cleaning up dead leaves can help prevent future infections.  This article lists some other common lilac problems and might be helpful if your symptoms don’t match those of Pseudocercospora:

https://extension.illinois.edu/blogs/good-growing/2021-08-05-whats-wrong-my-lilac-summer-foliar-diseases-lilac

If you want a confirmation of the problem, you can send a branch sample to the Ohio State University diagnostic lab (for a fee):

https://ppdc.osu.edu/submit-sample

Please let me know if you have further questions.