Hello,
as of today: this prompt comes from Outlook itself, not from PDF24. It appears because PDF24 Reader is now the program that opens your PDF files. As you noticed, the checkbox in the prompt cannot be changed there.
An administrator in our German forum described how they switched the prompt off for PDF24 Reader with the registry value EditFlags of the file type PDF24.Reader: https://help.pdf24.org/de/?p=17972 (we have not tested this ourselves).
Until now, an update of PDF24 removed this setting again. With the next version, the PDF24 installer (EXE setup) keeps it when PDF24 is updated.