The change in behaviour to the OnLevelInit forward params isn't obvious
when compiling a plugin, deprecate it to make it a lot more obvious that
something has changed.
Some plugins rely just on the timing of OnLevelInit rather than doing
anything with the entity lump, for these plugins offer a new OnMapInit
forward that is implemented in core rather than sdkhooks. If / when we
offer a new entity lump manipulation API in the future this'll be the
forward where it can be used to make changes.