IIR, as currently implemented, you still download and validate it. Assumeutxo just lets you get a node up and running instantly, while core does the actual validation on background.
The plan though is to allow a "don't bother" option that doesn't download at all. I haven't seen any talk on this direction you mentioned though. What would be the use case for that?
In most cases I understand that the primary bottleneck to bootstrapping a node is the TX verification.
IBD would happen much quicker if validation could be skipped, in favour of instantiating a specified UTXO set.
If we skip downloading the blockchain data altogether, we would only be able to operate as a pruned node. Pruned nodes lack a lot of functionality, like the ability to robustly support a Lightning node, running on top.
If assumeUTXO lets you get a functional node up and running, while the IBD takes place in the background, that's a good step in the right direction.
If the provision of a UTXO set could in some way negate the necessity for validation, as part of the IBD, that would be even better....
Well, I think it's pretty possible to do this, I even dare to say, trivial. You'll need to build indexes like txindex, but this is relatively cheap.
My curiosity was around why using ln on a pruned node isn't robust. It should make no difference, since ln node doesn't look for historical transactions, only UTXOs
