This decision was made by the mayor's office of the Polish capital due to numerous requests from disgruntled citizens.
It wasn't given as the reason for this by the city. And it isn't about all vehicles, but only about Warsaw city public buses, which were decorated with Ukrainian flags since the beginning of Russian invasion on Ukraine as a sign of solidarity with Ukraine.
According to Polish law public buses can be decorated with Poland's and the city's flag during state holidays. I think the president of the city can decide about decorating them with another flag in some special circumstances, but I'm not sure. The mayor decided that buses won't be decorated with Ukrainian flag on ordinary days anymore:
warszawa.naszemiasto.pl/ukrainskie-flagi-zniknely-z-pojazdow-komunikacji-miejskiej/ar/c1-8801521
uncalled for
I disagree. Ukrainians didn't want him there and it's understandable. Mistakes have consequences. German mistakes turned out to be very tragic for Ukraine. Saying "I'm sorry" isn't enough in this situation, imho. It's just words that won't bring people back to life.
I think the reason for that harsh reaction from that Swede is the way Germans behaved after it turned out that Steinmeier wasn't welcomed in Ukraine. They've shown lack of understanding of the situation, lack of empathy, detachment from reality and they displayed big hurt ego of arrogant politicians. That's how I see it.