If you are thinning it out you have nothing to be concerned about.

No I am not asking about thinning. That would be a result of removing canes. I am asking about “lightly shearing.” An overall haircut.

I wouldn't shear a lilac myself.

Nor would I. I’d only shear shrubs with dense foliage, thin twigs and small leaves, either to make a hedge or topiary. I would not usually shear a free standing shrub. If a subject would be hard work to shear by hand ( ie not with power tools) it probably shouldn’t be sheared at all imo but rather pruned correctly with secateurs. If you took 6 inches off lilac all over you’d end up with a twiggy mass. Prune individual branches and take them back to a suitable junction. You can dead head if you can be bothered.