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I feel like this is only really an issue with the new games and a side effect of the internet. In Gen 2, for example, there are NPCs who explicitly tell you all of the Pokemon with 'weird' evolution methods - Haunter/Machoke/Kadabra/Graveler has to be traded, Porygon has to be traded while holding the Up-Grade, Eevee needs to level up with high friendship, etc. Even in Gen 3, the most obscure evolution method - Feebas to Milotic - is something you'd be able to intuit just from knowing the lore / dex entries of both Pokemon plus the actual game mechanics.
Gen 4 started getting a bit nuts with stuff like 'level up a Pokemon while it's holding a specific BP-only item at a certain time of day,' or 'level up a Pokemon while it knows a move it doesn't normally learn,' and then never explaining it anywhere in the game. Gens 5-7 were a bit more of a return to sanity - the most obscure methods are either directly explained in-game (karrablast/shelmet) or are something that could feasibly be intuited (inkay).
Gen 8 is when the shit hit the fan and there was shit like 'take exactly 49 points of damage (from a direct attack, chip doesn't count) without healing and then walk to a specific location in the overworld' that seems like it's explicitly designed to get people to look it up. Maybe it's part of a clever bid to get people to look up Pokemon online and get more advertisements/recommendations for it
