If you see this symbol on your equipment, this is a bad sign...
That is silly. There is plenty of good quality double insulated equipment, and there are plenty of applications where this "good enough" or even preferred above grounded equipment.
Connecting the secondary of a transformer to protective earth is not done for low voltage transfromers. If you want a lab power supply, the secondary side is usually floating. As a result you can put multiple supplies in series, and then call any of the three terminals "ground", so with two 15V power supplies you can use it for either +30V + / - 15V or -30V. This would not be possible if the secondary winding would be connected to protective earth. Such a connection would also create strange results if the secondary has a bridge rectifier.
Connecting the secondary of a transformer to PE is sort of common for high voltage transformers. Your microwave oven which has 2kV output probably has it's secondary winding directly connected to PE. As a result, that side of the transformer is guaranteed to have a low voltage, and the high voltage with it's several kV potential is the only side that needs thick insulation, warning signs etc.