Ah ok. Some of my components use Mechanical 1 as silkscreens, would I need to remove these/change layers if I were to make the outline using Mechanical 1 (not just for printing but also for Gerber generation to send to the fab)?
Altium provides dedicated layers for silkscreen: Bottom Overlay and Top Overlay. These are what get rendered in 3D view, and they get the standard GTO and GBO gerber extensions by default, so it's a good idea to use them. In AD18 and earlier you had to use Mechanical layers for basically everything except copper, silk, and paste. There were conventions as far as which layers to use for which purpose, but there was nothing to really enforce those conventions. AD19 now has explicit component layer pairs for 3D Bodies, courtyard, designator, etc, which should make things a bit easier to deal with, at least once component libraries get updated to use them.
The boar dcontour is defined in board outline. it is no longer a separate layer.
What do you mean by this? Is there some new way to define a board outline without using the standard line/arc primitives in a mechanical layer?
(I know you can define from a 3D body, but I don't particularly like the lack of control over the mechanical interface this provides. Also obviously Altium represents the board outline internally separately from the primitives that were used to define it, but I don't know of a way to initially define the board outline without some sort of mechanical primitives (D,S,D).)