I think what happened is Properties were introduced in ARCHICAD 20 and those take the place of these custom parameters. They are a more general solution and a Property will be available for all Doors or Windows, not only the Doors or Windows where such a parameter exists.
So I guess the GDL developers have removed these "User Defined 1... 5" parameters to remove the redundancy.
So you should not use IFC Properties for this in the future, you should define Properties for this purpose.
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