I've almost never reduced a tip for service in the US, it really has to be clearly intentionally crappy service from the actual wait staffer and at an expensive restaurant for me to consider reducing a tip.
I’ve done it
once. Left a penny tip after an hotel’s Easter brunch because of the way we were treated at the end of our meal.
Back in the late 1980s, I invited 3 friends to the brunch, and we got the last reservation slot. It wasn’t big- a single room about the size of a medium-sized restaurant- but it was nice. We had a good time up until we noticed we were the last table, and decided to get our desserts.
We came back to the table, and found it had been bussed. Not only that, the ladies’ purses had been picked up and taken to the register.
To be 100% clear, we were never out of their sight. We were clearly visible at the dessert table from the register, and our table was in an unobstructed straight line between those points. We were 15-20’ away from our table at most.
They had not asked us to speed up or leave, and obviously, the food was still on the buffet. They just cleared our table and took our possessions.
I talked to the person in charge, who told me the buffet had ended. I replied nobody had told us, the food was still out, and we were clearly visible at the dessert table. I informed him we’d been there just over an hour, and if they didn’t want people eating as late as we were, they shouldn’t have had reservations available for the time we booked. I demanded the return of the purses, fresh beverages, and the utensils & napkins needed to eat our desserts. I paid the bill- including the penny tip- while they did so.
Why a penny? Because I wanted them to know I was pissed off at our treatment AND that I hadn’t
forgotten to tip.
I never went back to a holiday brunch at any hotel in that chain ever again.
(FWIW, my usual tipping practice is 15-20%, and 25% for special events with above average service. And I’ve tipped over 100% at least once as well.)