>Is this a global ban on using HEVC to stream UHD?
Since this is in German Court I would assume it is for Germany only.
>What does Netflix use to stream 1080p?
Resolution has little to do with Codec. So you could have 1080P in both HEVC / H.265 and AVC / H.264. I believe Netflix provides both encoding in case one isn't supported. Although 4K may straightly be HEVC only. Not because AVC; the codec dont support 4K but more of a limitation of hardware decoder and encoding inefficiency.
>Assuming Netflix turned HEVC off tomorrow how would UHD streams happen on a random smart tv from almost a decade ago?
They may only be watching HD instead. Or Higher Quality of HD encoding with higher bitrate. Which is perfectly doable for Netflix due to their comparatively low amount of content. Certainly impossible for something like Youtube where you have a constant influx of new uploads.