Whenever you need a swap convention, just google ISDA.
The floating leg of swaps follows the euro money market convention.
For the fixed leg, ISDA has concluded after consulting members that 30/360 (annual) is the most appropriate basis for calculating interest accrual on fixed payments for euro-denominated swaps.
The reason for adopting a 30/360 (annual) day count fraction is that back when this was decided, existing DEM and ECU swaps quoted on this basis and that the emerging practice for the euro is to follow the same approach, as reflected on broker screens.
That said, these swaps should only exist for legacy reason now after LIBOR cessation.