Is liquid helium an absolutely “pure liquid”?

The purity of liquid helium was already a subject of much interest in the 1970's due
to its importance, for example, in the efficiency of the production of ultra cold neutrons.
In the last decades a recurrent worldwide issue, the flow impedance blocking
in helium evaporation cryostats and dilution refrigerators (T < 4.2 K), also ascribed
to the presence of impurities in liquid helium, has become more and more frequent.
The problem is causing tremendous helium losses and delays in research at low temperatures,
all over the world.