There's a built in "night shift", but that just changes the colour temperature, it doesn't make everything monochrome red.

No, that's (part of) what Color Filters is for.

iOS has also had “Night Shift” for several years. The parent is talking about a full on red colour filter, like astronomers might use.

Kinda feels like you could’ve done more of a cursory glance to see what functionality was actually being talked about before going for the “Android already does this!!” comment.