Currently, if a sheet needs an extension listed in its <dependencies>, a dialog is shown that allows to install them.
But if the install is rejected, or the extension is not in the <dependencies>, or those extensions are disabled, then custom regions of absent/disabled extensions are simply absent from the sheet.
The proposal is to output rectangle with some text in place of those regions:
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if the XML of the region contains a fallback text in a dedicated tag, then output this text;
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otherwise, output something like "Extension "XYZ" for this region is not installed" (variant: "is disabled").
One place where an extension-defined text makes sense is in Writer extension, where it could output its unformatted version of text in case the extension is absent.
Also, it could make sense to output cached image as a fallback in case the graphic region provides it in XML.
Anyway, if a fallback is used, it is desirable that this is made apparent to user.