suppose a Complex terrain is created by combining multiple terrains. At some point thereafter if one of the individual terrains is deleted, if the "Edit Complex Model" tool is used the deleted terrain is simply gone. There's no indication that it was ever part of the complex initially.

I'd like to propose that the "Edit Complex Model" tool will remember all the terrains that were used to create it and will show any terrains it can't find with red letters (to be consistent with how unresolved reference files work). This could be a helpful way to let designers know that something was previously part of the complex that is no longer available.