I was told not to trim the old flowers off lilacs, because I might cut off next year’s flower buds which had formed over the summer.
Any parts you cut back hard won’t flower for at least two years, if then.
OTOH....
I’m in eastern Ontario, and I have noticed my Lilac is bereft of flowers, except on the top this year. Maybe they need a colder winter?
That is true about lilacs blooming on old wood. That is why I said to trim an old bush half way down and wait for 2 years for new blooms. If we couldn’t cut the flowers for bouquets we could never use them in flower arrangements! And that is why you wait until after the bush blooms this year.
My lilacs are now about 18 feet tall and barely have any flowers any more.
BTW, I have 2 new bushes that I got from the Arbor Day Foundation. I planted them as 12” tall sticks 2 years ago and they are now 8 ft. tall and blooming for the first time. But, they’ll have to be pruned in order to shape them into a bush. Rght now, they are just these tall, spindly things. I planted them in the perimeter of my veggie garden for “storage” because I didn’t know where I wanted to put them forever. Got to make a decision soon.