As a European living in Europe I am more of the impression that anti-union sentiments are mainly widespread in the USA. But then again I haven’t really discussed unionization with anyone either. I just see more negativity towards it in public from Americans than from others.

In East Europe anti-union feelings are strong among the people that lived under the communist regimes (~ 45 years or older). Unions were widespread at that time and the negatives were very visible, hard to forget the huge negative influence they had in the period of 1990-2000 when the transition of these countries was slowed down or sabotaged by unions.

I don't think that's blanket true. What about Solidarnosc?

It was well regarded at some point, but the current membership numbers (4% of what it was at peak) is showing that the post-1990 evolution was not great. Providing support for closing stores on Sunday is not the purpose of a worker union.

Source?

Really everyone that I know in that age range. I am living in Eastern Europe and I worked in several countries in the region.

I know many software engineers who like unions actually, they tend to work at larger companies. There's definitely a lot of affinity for libertarianism I've observed amongst software engineers. I try not to psychoanalyze.