Being afraid that the seed phrase won't be valid in the future would be the same for wallet import formatted private keys. The answer is yes, it's safe as long as you have the function to derive the keys. Electrum's derivation function can be found in github, but the method is written on multiple projects. For example, Coding Enthusiast's Denovo has implemented it too. (ElectrumMnemonic.cs)

So even if electrum shut down their website and removed all of their repositories, you could still derive your private keys by using the csharp script above, properly.