As discussed earlier I received a long list of warnings and errors when running the premigration check. The next entries in my list are a couple of warnings of this type:
I assume that this warning will only hit a very small set of customers as it only affects customers who have implemented custom placeholder definitions. As most customers are using the standard placeholder definitions and "only" implement custom placeholder controls.
In my case the event details showed exactly which custom placeholder definition was affected and how it will become migrated:
The error message is not fully correct as it asks the user to replace the placeholders with custom field controls. Actually a custom field controls is the MOSS 2007 equivalient of a custom placeholder control and not of a placeholder object.
In this situation I will most likely implement a custom field type (which is the MOSS 2007 equivalent for a custom placeholder definition) and migrate the content with some code from the plain text field into a field of my custom field type.