Azrail93
A lot of conditional evolutions have been likely found out with the obvious elemental stones being used to evolve Pokémon of the same type being the most obvious one. Happiny needing an Oval stone to evolve is something Chansey knew and it is likely the Joy clan know but others knowing is a maybe. The Joy and Jenny or Joi and Junsar for the Japanese equivalent also likely have male counterparts as they likely need spouses to have children and they should have male children on occasion as well unless during one of the first three civilization they made a purely female lineage. Anyways move conditional or time conditional evolutions are likely easy to figure out such as aipom needing to know double slap. Trade evolution might be figured out and people would then likely wish to replicate trade evolution without giving away their own Pokémon and then they would try and figure out how these Pokémon evolve naturally in the wild and see if it could be replicated/ increased potential more. Others such as Milcery and Applin having evolutionary conditions related to the food they resemble would be simple to discover. Pokémon that evolve by trading and with an extra item are likely to be super hard to figure out especially the karablast/ shelmet cross trade. Most cross trades with an item involve an item of compatible type so Clampearl's evolutionary items are likely rare as they have deep sea in their name. Politoad/Slowking are also likely rare as they involve a king's rock a rare collector's item that allow for an optional alternative evolution to two species that can already evolve by basic means already and likely only had slowking being investigated as it's appearnce made it obvious it was related to slowpoke with politoad likely needing more study by people unless frogs are still around. Spiritzee and Swirlix are maybes on if they are found out as sachets and whipped dream are luxury goods that multiple pokemon like. Porygon if they exist need two separate item evolutions and are thus really likely to be hard to evolve. That just leaves Eevee's Glaceon/Leafon only happening near the mossy/icy rock and not with an ice/leaf stone. This will likely be difficult to discover or replicate artificially even if found. Finally Feebas is likely the weirdest as far as people would be concerned as it is an unattractive Pokémon that only evolves when it thinks it is beautiful. Prism scale trade evolution may be a thing or prism scales remind them of Milotic and they feel better holding them and again run into the problem of prism scales being rare beautiful collectors items. So those are traditional Pokémon with evolution conditions that would likely be the least understood or show up the least frequently.