Hi!

This looks like a fungal leaf spot, likely Pseudocercospora. While this can be confused with a bacterial leaf spot, Pseudomonas spp., this looks like it's due to a fungal infection to me. I think we'd see more angular looking spots if it was bacterial. The only way to know for sure would be to have the tissue tested in the lab. If you continue to have issues with this after implementing some of the management practices below, you may want to send a sample in. https://www.canr.msu.edu/pestid/index

Luckily, since you are doing some pruning that will help thin things out and increase air circulation. The fungal leaf spot is mostly a cosmetic issue and doesn't require any treatment. It's been very humid, which can bring on fungal diseases like this.

Here are some best management practices and a few resources on lilacs.

• prune off infected leaves and dead twigs during the winter or dry summer months to prevent the spread of the disease spores. (you can still do this now, just try to do on a dry day when it's a bit cooler)
• Keep lilacs well-spaced and properly pruned to encourage good air circulation around the foliage. Leaves that dry quickly after rain or dew are less vulnerable to infection.
• Collect and dispose of infected fallen leaves in autumn.

https://extension.umd.edu/resource/lilac-identify-and-manage-problems/

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

https://yardandgarden.extension.iastate.edu/encyclopedia/lilac-pseudocercospora-leaf-spot