The term "Pseudo-legendary Pokémon" is a fan term created by the community to define Pokémon with those characteristics. In the Japanese communities, they're referred to as the 600 Club (600族).
These terms have never been used officially, however it is very likely that the devs are aware of the classification. The topic has never formally been addressed.
However, in Gen 9, the term "Powerhouse Pokemon" was used to describe the bunch. It is presumed that Powerhouse Pokémon is the new official term, given by its usage in Gen 9, as well as its line on the Pokémon Center website.
TL;DR It seems like "Powerhouse Pokemon" May become the official term, but I think Pseudo-Legendary is official in our hearts
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