The advice re allergies has changed in recent years, the general 6 month recommendation has been around for quite some time, however, though obviously some health professionals don't always pass on the official advice (though they're supposed to) and obviously individual parents can do whatever they wish in any case

The timing of the introduction of possible allergen foods is a tricky one, largely due to the lack of good data. In countries like Australia, most babies receive solids and/or formula before 6 months (and many before 4 months) so it's really hard to say one way or the other because we have so few babies who reach the 6 month mark exclusively breastfed, let alone any further than that.

We do know that introducing anything other than breastmilk before 4 months significantly increases the risks of allergies. We also know that weaning from the breast while introducing solids increases the risks of allergies - so continuing to breastfeed while you're introducing appears to offer some protection.