There are countless open source repositories, libraries, packages, etc., which will turn your seed phrase in to private keys. There is no way that they could all be shut down short of a global adversary censoring the entire internet, in which case bitcoin would be worthless anyway. And even then, the process for turning a seed phrase in to individual private keys isn't exactly top secret or even that difficult. It is essentially just a bunch of hash functions repeated multiple times. Even a complete newbie with a bit of determination would be able to figure it out without much difficulty.

If you are really concerned, then download something like https://iancoleman.io/bip39/ from the linked GitHub repository and store it on your computer, a couple of USB drives and CDs. That way, you know that you always have access to a piece of software which will recover your seed phrase for you if needed.