The gist is here, that the complete first four season are on YouTube for free and the 5th is being added as we speak? (200+ episodes)
https://www.youtube.com/@OfficialPoke%CC%81monTV/playlists
There was nether the expectation with streaming that third party content doesn't rotate.
If you want a bit more persistent access you can buy them on Apple TV (Season 1-5 and 10-25)
Oh Boy, Pokemon is really not the example I would bring up here, when the aim is completeness on official channels:
https://en.wikipedia.org/wiki/Pok%C3%A9mon_episodes_removed_...