Standard beer may have quite a short shelf life, however some is brewed specially to have a long shelf life, IPA is a good example made so it could be taken to India by sailing ships and still be OK when it arrived 6 months or more latter, so this should last in an unopened keg for at least a year.
Lager should last even longer, the whole idea of the French process was it would last longer than more traditional methods so should last longer than IPA.
Again the higher the ABV the longer it will last.
So bought three beers from Lidi, all with best by date of around a year, one a bitter at 3.6 ABV% one a lager 4% ABV the the last a PILS 4.5% ABV this does not seem to make sense.
Even my kit home brew which should be the worst beer to keep since using tap water, keeps for a year often two years before I drink it, so why such a short self life on commercial beers?