There was a post here about 6 years ago for a site that calculated line of site for any two points on a map with both the max line of sight and 2D cross sectional view of the terrain difference between the two points. I haven't been able to find it since 2020, but it was awesome.
Could it be this? https://www.udeuschle.de/panoramas/makepanoramas.htm
No, it used a much heavier JS interface and IIRC it also used Google Earth. That is a nice tool, thanks for sharing it.
It was https://caltopo.com/
https://www.heywhatsthat.com/ is another bookmark that I had lost to time.
heywhatsthat?
I think that was it, thank you for finding it again!
Actually, I was thinking of https://caltopo.com/map.html but your site led me to it.