What does this error message mean?
You tried to create a BDB wallet, which is the old type of wallet which is deprecated.
For context, Bitcoin Core supports two types of wallets:
- The old type ("BDB" wallets, named after the database format they use)
- The new type ("descriptor" wallets, or "sqlite" wallets)
BDB wallets were the only supported type until Bitcoin Core 0.20.0. Since 0.21.0, descriptor wallets have been gradually introduced as a new, more modern wallet format. The plan is that in some future version, perhaps 29.0 or 30.0, it won't be possible to use BDB wallets anymore, which is why since 26.0 the warning you're seeing is present.
If and when this removal of BDB support happens, conversion tools will be provided to convert BDB wallets to descriptor wallets, so no funds will be lost or anything like that. However, since some people still use (very) old versions of Bitcoin Core, after such a conversion, they may be unable to use the wallet in old versions anymore, and they may need new backups, which is why users need to be told that it's happening at all. Furthermore, the commands that work on these wallets differs a bit, which means guides, or even other software on top, may not work on descriptor wallets until it is updated to support these.
You probably just want to create a descriptor wallet (the 6th argument is changed from false to `true):
bitcoin-cli createwallet name false false password false true true
What is a BDB wallet?
A wallet.dat file that uses the Berkeley DataBase format, and with a contents structured in a way that (very) old Bitcoin Core versions can read.
What does it mean that its creation is "deprecated?"
It means that creating such BDB wallet files is still possible in the version you have, but it likely won't be possible anymore in some future version (which is not yet released). Since you may have business practices that rely on using BDB wallet files, it is essential that you're being made aware of the fact that you're doing something that will stop working in the future.
If you really want to create BDB wallet files still, you will need to run bitcoind with the -deprecatedrpc=create_bdb command-line option, to tell it you're aware you're using a feature that is scheduled for removal.
What is this future release?
Maybe 29.0, or 30.0. It doesn't exist yet.
Is the bitcoin core as downloaded a non-updated thing?
Bitcoin Core (deliberately) has no auto-update feature. It'll update whenever you choose to install a new version. You can choose to keep running the current one forever, and BDB will keep working then. However, that also means you won't get bugfixes, new features that get added, or protocol improvements that happen.
Or is it just referring to a future update that will take place once enough nodes agree to it?
Other nodes have absolutely nothing to do with this. Bitcoin Core updates whenever you update it. There are protocol features that depend on multiple nodes speaking the new version, but the wallet file format is a purely local decision that doesn't affect other nodes.
Once a wallet is created, where is its info stored on my computer?
In a file in the data directory for your node, typically with a name "wallet.dat", but it can be named anything. Make backups of it.
Is it fully functional from the daemon?
Yes.
