Hi Piotr,
If your partner account exists prior to April 1st 2025, you’ll be able to create custom apps that use our REST API after April 2025.
However, this does not apply to the REST Product API deprecation however (which is where the confusion might have been). The REST API endpoints for /products and /variants will be deprecated for custom apps on April 1st 2025.
But if your creating custom apps that don’t rely on /products and /variants REST endpoints you can create them after April 1st, in the same way they work today.