Just to add to the testing, when I tried your to open your scene in 1.7.4, I get no Legacy checkbox (opened in Max 2018 3 and 4) as you report.
If I load the scene using Corona 1.7.2 and then save it without making any changes (just saving to a different file name) and then open the resaved version it in 1.7.4, I then do see the Legacy checkbox.
If I load the scene in 1.7.4 and resave it, this resave still does not show the Legacy checkbox on loading - so only if I resave from 1.7.2 does it fix it.
I created a simple test scene in with 1.7.2 and saved it, but it opened fine in 1.7.3 - so I haven't found any way to duplicate saving a scene where the Legacy checkbox doesn't work, unfortunately. I did try saving a scene with 1.7.2 and Max 2018 patch 3 and opening in Max 2018 patch 4 , but that didn't duplicate the issue either.