This answer is based on the assumption that Mew's answer on this (https://pokemondb.net/pokebase/49649/are-there-any-pokemon-episodes-with-no-pikachu) is a correct list of all episodes without Pikachu up until that point.
Since then there have been 303 new episodes. All of which I have gone through on Bulbapedia in order to find out whether or not Pikachu made an appearance. Bulbapedia has a list of every character appearance on every episode, I checked that list on all 303 episodes.
Pikachu makes an appearance in every single episode of season 15, 16, 17, 18, 20 and 21.
In Season 19 however there is an episode called "The Legend of X, Y and Z!" in which both Ash and Pikachu are absent. Which seems to be the only time since Mew's answer was written that Pikachu doesn't appear in an episode of the anime.
That means the list would be:
- The Legend of Thunder (Part 1)
- The Legend of Thunder (Part 2)
- The Legend of Thunder (Part 3)
- A Family That Battles Together Stays Together!
- Cerulean Blues
- We're No Angels!
- Showdown at the Oak Corral
- The Blue Badge of Courage
- A Date With Delcatty
- Celebi and Joy!
- Training Daze
- Journey to the Starting Line!
- Putting The Air Back In Aerodactyl!
- Luvdisc Is A Many Splendored Thing!
- Those Darn Electabuzz!
- The Search for the Legend
- Of Meowth and Pokémon
- Trouble in Big Town
- Big Meowth, Little Dreams
- The Legend of X, Y and Z!
Source: The Bulbapedia page for every season 15, 16, 17, 18, 19, 20, 21. For example (https://bulbapedia.bulbagarden.net/wiki/BW049)