When has the presence of a pre-evolution ever not lead to its evolution being in the same game.
LGPE didn't have any evolutions that weren't in Gen 1, so Eevee didn't evolve into Espeon, Umbreon, Glaceon, Leafeon or Sylveon. Magneton didn't evolve into Magnezone. Scyther didn't evolve into Scizor. Tangela didn't evolve into Tangrowth. None of the Baby Pokemon were in the game either.
BDSP didn't allow Eevee to evolve into Sylveon. None of the Regional Variant Pokemon could be sent or be caught in this game either despite their original forms being available.
PLA didn't allow for non-hisuian versions of Pokemon other than Sneasle to be in the game so you could not get any of the Starter's original evolutions, Original Braviary, Avalugg, Qwilfish, Voltorb, Electrode, Growlithe, Arcanine, Basculins, Zorua, Zoroark, Sliggoo, Goodra or Liligant.
The precedent has existed since Gen 7 and was compounded in Gen 8. They have the capability to prevent a Pokémon with a regional variant or cross-gen evolutions to not allow those to appear if they disable them and we know they can disable specific forms of Pokemon from being available.
We are at an age in Pokemon where they can and have the capability to completely lock a Pokemon evolutionary line if they want to for any reason they choose. If they decide they don't want Perserker in the next game, they can make it so only Kantonian Meowth is available so that you cannot get either variant or simply lock Galarian Meowth and allow Alolan Meowth.
Next Gen they could have more Cross-Gen evolutions and lock the original evolutions out simply because they can. This is a possibility now. For the sake of how Pokemon Power wants to make their list, this is a valid concern even if you or I disagree with how they are conducting the creation of their list.