The Pokémon Company announced on September 6th, 2023 that the Pokémon TCG Card Dex app will be removed from the App Store and Google Play, and services will be deactivated, and data will be lost on September 20th, 2023. A Pokémon Support page answered some questions about the end of service for the app.

Will my data be transferrable to another game or service?

Pokémon TCG Card Dex data will not be exportable or transferrable in any way. On September 20, 2023, following the app’s removal from the App Store and Google Play store, all data will be lost.

Will Scarlet & Violet—151 be available in the Pokémon TCG Card Dex app?

The Pokémon TCG: Scarlet & Violet—151 expansion will not be coming to the Pokémon TCG Card Dex app. Scarlet & Violet—Obsidian Flames was the final expansion to release on the Pokémon TCG Card Dex app.

How will I track my Pokémon TCG collection and learn about the latest cards?

While the Pokémon TCG Card Dex app will be sunsetting, players will still be able to search the Trading Card Database to learn about cards from past and current expansions. Players will also be able to build a digital Pokémon TCG collection in Pokémon TCG Live by redeeming the code cards included with physical Pokémon TCG products.

Why did you decide to sunset the Pokémon TCG Card Dex app?

The decision to sunset the Pokémon TCG Card Dex app was made to dedicate our full resources to the ongoing development of Pokémon TCG Live, ensuring that we’re devoting our efforts to providing players with the best possible digital Pokémon TCG experience.

Source: https://support.pokemon.com/hc/en-us/articles/18773957465748/