so would that mean that a paper wallet can't be done anymore?
Bitcoin Core has never supported paper wallets.
In any case, single key paper wallets will work with descriptor wallets since they are single private keys. A HD seed is not that.
what are other solutions that do not imply saving a file?
Exporting and storing the descriptors of a descriptor wallet.
plus, i read the legacy wallet option is going to be removed from the bitcoin core, so that wouldn't be a long term solution.
Yes, it will be. However there are plans to reintroduce sethdseed in some form so you could still store just the seed. But that's still not exactly sufficient as you don't have information about derivation paths and scripts. Descriptors contain all of that information, so I recommend you store those.
consider i want to do something that would last forever. what would you do?
Nothing is guaranteed to last forever, but something that follows published standards is certainly more likely. The legacy wallet's HD seed stuff most certainly does not follow the existing standards.
I would suggest using a descriptor wallet and writing down the descriptors. While it's a lot more verbose, it is also much more complete and will not rely on guesswork in the future.
You could also just do a standard BIP 39 mnemonic, but keep in mind that these do not contain derivation path or script information, and they are not (currently) supported by Bitcoin Core.