If I were in their shoes before the war, it’s much better to engrave the phrases in indestructible metal bars of any kind as it is resistant to fire, water or any calamity.
The detonation of a war bomb is incredibly powerful, and even if you store something in metallic form, I can assure you it will suffer physical damage, with the mildest possibility being that your seed phrase becomes illegible. In wartime conditions, it's advisable to leave the battlefield when feasible, as it's a matter of safety and also safeguarding our assets.
If you haven't considered how to securely store your phrase online, you're putting yourself and your assets at risk during such wartime scenarios. However, if you store it online, you shouldn't have to worry about your phrase once the war is over.
Note: Don't store the phrase in plain text. You can use a puzzle technique to make sure the phrase you create can only be deciphered by you. I believe that's a safer approach.