Slashed Pokémon are one of the new anti-cheating measures deployed in June 2017, right after the release of Gym Rework.

Trainers using third party apps and services that promote cheating, violate Terms of Service or break Trainer code of conduct are flagged and their rare Pokémon become “slashed”.

The term “slashed Pokémon” comes directly from Niantic, as their reddit support staff has shared the following on this topic:

With the announcement of Raid Battles and the new battle features, we are staying true on our commitment to ensuring that Pokémon GO continues to be a fun and fair experience for all Trainers. Starting today, Pokémon caught using third-party services that circumvent normal gameplay will appear marked with a slash in the inventory and may not behave as expected. We are humbled by the excitement for all the new features we announced yesterday.This is one small part of our continued commitment to maintaining the integrity of our community and delivering an amazing Pokémon GO experience. – source

What we know about the Slash mechanism?

We have confirmed the following:

  • Slashed Pokémon can’t participate in Gym Battles
  • Slashed Pokémon can’t be deployed as Gym Defenders
  • The “Slash” is no longer removed upon evolution
  • You get no Candy for transferring Slashed Pokémon

The following apps and services are known to cause slash to appear:

  • IV Checkers that require you to enter account information
    • IV Checkers that use screenshots are safe
  • Tracking apps and websites that require you to enter account information
  • Apps that fake GPS (GPS Joystick, FlyGPS, GPSGO, etc…)

We have also confirmed that Slashed Pokémon follow the same bracket logic introduced in earlier shadowban systems. The following Pokémon are most likely to be slashed:

Evolved Pokemon, starters, Caterpie, Weedle, Spearow, Clefairy, Vulpix, Jigglypuff, Venonat, Mankey, Growlithe, Abra, Slowpoke, Shellder, Gastly, Onix, Drowzee, Voltorb, Koffing, Chansey, Tangela, Horsea, Mr. Mime, Scyther, Magmar, Lapras, Eevee, Porygon, Omanyte, Kabuto, Aerodactyl, Snorlax, Dratini, Hoothoot, Chinchou, Mareep, Sudowoodo, Aipom, Yanma, Unown, Wobbuffet, Girafarig, Shuckle, Sneasel, Teddiursa, Swinub, Remoraid, Houndour, Stantler, Larvitar – source