Hello. Can I make shortcuts enroute when flying IFR or should I fly the flightplan route on unicom?
Yes, you can. When you talk to (enroute) ATC again, tell them in the initial call what waypoint you are navigating to and be prepared to get recleared to another point by ATC, potentially back on your planned route.
Is it applicable only in countries with free routing airspace?
No, you can do it almost anywhere. I would NOT do it when crossing the North Atlantic Ocean and when you are following a planned North Atlantic Track.
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thank you
If you do it, I would also advise to keep it reasonable and to keep in mind both the airspace structure and who’s online further along the route. Especially airways don’t just serve a purpose of getting aircraft from A to B along a standard route, but also helps avoid special airspace and/or terrain, there are agreed upon procedures for handovers between sectors etc.
Speaking as a European controller, where airspaces are fairly small and complicated, I regularly pick up pilots from UNICOM that have given themselves a direct of hundreds of miles, across multiple sectors, often several countries, and it’s always interesting to see them go straight against the regular flow of traffic. 
This sounds amazing mate as I found these insights very much useful and informative to be honest.