"Variable bitrate" encoding is a thing, which allows chunks of a file to be encoded at a higher or lower bitrate depending on how much is happening. I don't know exactly how it happens, but I assume that each chunk is determined by duration or size, while Netflix's new method determines each chunk by the content.

In order for this to be the default, you'd need either humans, or pattern recognition algorithms to identify the chunks. You also need to quantify how by how much each chunk needs its encoding parameters tweaked.

Monetary costs aside, that's increasing complexity of your pipeline with relatively small gains. I'd bet that more companies will start looking at similar approaches now that 4K HDR (and 8K) are becoming more common. Probably not worth the R&D for 1080p, but we'll see tricks like this start to trickle down, I'm sure.

It's highly unlikely we'll see anything similar in FOSS tools in the near future.