Windows loves wasting time, and this is one of its stupid habits.
I only have to create a junk file called junk.jnk, and even if I immediately delete it Windows will obsess about *.jnk,
wanting to know what to do with it should I double click on it.
The same stupidity occurs with *.odt, and in fact with any Open Office extension,
because I use Portable Apps version of Open Office to conserve disk space and avoid registry junk.
When I launch Open Office it records the registry keys it will take over and then I can access *.odt etc,
Windows immediately takes notice and records somewhere the fact that *.odt is a significant extension,
and that a double click should use Open Office to process that file.
At that stage Windows knows of this extension, and it is NOT unused.
When I close Open Office its tidy-up procedure is to :-
move relevant things into its own *.ini file ;
restore the system back as it was, which includes the keys it had taken over.
Unfortunately nosey parker Windows still remembers that *.odt is a relevant extension,
but it has forgotten what to do with it.
I have no qualms at all about ill effects of deleting unused extensions,
but I am always (almost) alert to any possible evidence of a Windows hiccup to fix, or of a malware intrusion,
so I simply scan the lists of files and registry keys selected for deletion to see if something new has happened,
and then they get purged.
Alan