While the terms "pseudo-legendary" and "600 club" are fanmade terms, they have always been used to address what is essentially a very clear and deliberate pattern of Pokemon that Game Freak has designed and implemented into the games. They have just never coined an official terminology for the particular group of Pokemon it addresses but the group of Dragonite, Tyranitar, Salamence, Metagross, Garchomp, Hydreigon, Goodra, Kommo-o, Dragapult, Hisuian Goodra (yeah I'm counting this separately), and Baxcalibur all align with a clear pattern with which at least one member is introduced each generation.
They clearly acknowledge those Pokemon as part of a specific group with each other with an official merchandise set called "Taiki-Bansei" (Late Bloomers) focused specifically on them. A pair of NPCs in Sun and Moon's Battle Tree has all of them as part of their potential roster alongside certain restricted legendaries. They are often version exclusives with each other, with Salamence and Tyranitar being version exclusives in Diamond and Pearl, Gible and Dratini being version-exclusive gifts in BW2, one special Trainer in USUM literally has Shelgon, Gabite, and Sliggoo as his team, Sword and Shield had Tyranitar and Hydreigon, Goodra and Kommo-o, and Salamence and Garchomp as version-exclusive pairs and four of them were in the Lake of Outrage alongside Dreepy as rare spawns. Scarlet and Violet has Tyranitar and Salamence as version exclusives once again alongside Hydreigon and Dragapult being direct version exclusive counterparts with each other. It's an implicitly acknowledged and intentional group of Pokemon.
The specific group in and of itself follow a distinct set of rules that all of them abide by:
- Three-stage evolutionary line
- Slow EXP Group (1,250,000 EXP to reach Level 100)
- Entire-line is self-contained in one generation
- Has the highest evolution level of all non-legendary Pokemon in that generation's roster.
- (Related to the above, evolution is entirely by level-up).
The evolution levels are actually a clear commonality, as you can see here when you put it like this, which is likely what inspired the term "Late Bloomer" for their merchandise set:
https://pastebin.com/PKdUMxpW
But yeah the group is part of a clear and deliberate pattern and there have been a pretty clear set of trends that they tended to follow.