100 nits is the number that matters
BT.709 assumes a reference white of 100 cd/m², set for a dim grading suite in 1990. Everything about SDR follows from that ceiling: a highlight cannot be brighter than diffuse white, so a sunlit window and a sheet of paper both land at the top of the range and both render as the same flat white.
Modern displays exceed 100 nits comfortably, and an SDR file shown on one is simply stretched. The relationships inside the picture do not change, because the encoding never carried more.
Where you will still meet it
Almost everywhere. Every non-HDR export, every screen recording, every phone clip shot with HDR turned off, most stock footage, and the SDR deliverable of nearly every finished project. If a file's color_transfer reads bt709, it is SDR, whatever the file is called and however bright it looks on your monitor.
The conversion target in reverse
Going the other way, from an HDR master down to BT.709, is where most washed-out results come from: the PQ curve has to be tone-mapped into the smaller range rather than simply reinterpreted. Converting HDR to SDR without the washed-out result covers the working methods.
Questions people ask
Is BT.709 the same as sRGB?
They share the same primaries and white point, so the colours line up, but their transfer curves differ: sRGB uses a piecewise curve near black and BT.709 does not. In practice they are treated as interchangeable for video, and the distinction matters mostly when compositing stills into a timeline.
How do I know if a file is BT.709?
Read the stream: color_primaries=bt709 with color_transfer=bt709. An empty or unset transfer field usually means the same thing, because BT.709 is what a decoder assumes when nothing says otherwise. Either way you have an SDR file.