Fascinating read! I grew up in the south-east of Europe and for me any shoes worn outside are just for that - outside. Same like coat.
When you go inside, you take them off. If at home, we have slippers. If visiting, unless the hosts offers or insists to keep them on, my family always take them off.
All this is for the hygienic reasons alone, then comfort and warmth, I cannot imagine wearing outdoor shoes indoors unless, as mentioned, the hosts insist.
Funnily, I never thought of bare feet as rude, nor had that perception ever come up in my surrounding and consequently made me think of the shoe etiquette.
I also never thought about slippers too much before my son started kindergarten back in England a few years ago. I was horrified when I realized the children spend the whole day in the same shoes, going out, staying indoors - and worse of all the parents also walked in the same shoes through the very same space where the children (babies!) are walking, crawling or generally laying on the floor. I know a healthy dose of exposure to bacteria is good for immunity, but that was too much. But I had to live with that. the reasons were, however, not cultural, but purely practical, the space was so tiny they struggled with decent wardrobe space and catering for shoes changes would be too much.
Now, in Germany, it is the complete opposite. The children change into slippers and when parents drop kids off or pick them up they go to the kids' areas, but they either change to guest slippers or put protective covers over their shoes and then walk in.