I'm not sure I agree with the deprecation strategy here. The whole point about extending MultiDict::InstanceSource and FileStorage::InstanceSource is that you want the additional taint steps to kick in. With these changes, if anyone else than us has done this (and I HIGHLY doubt this), their code would still run, but it would not work anymore (since the additional taint steps only kicks in if you use InstanceSourceApiNode).

I probably should have labeled these as INTERNAL: Do not use. when adding them 😞 but that's too late now. I suggest we either:

1. Remove all traces of MultiDict::InstanceSource and FileStorage::InstanceSource, and take on the risk breaking someones code they actually these.
2. commit to following our deprecation policy, such that the functionality you expect from MultiDict::InstanceSource and FileStorage::InstanceSource is still working even though they are deprecated.

I'm leaning towards option (1), since I think the chance of someone else than us having used these is 0%, and that keeping these deprecated versions functional is going to slow us down.

From internal discussion I learned that this way of deprecating this is ok. I'm personally not 100% in agreement, but I'm going to try and not make too much of a fuzz about it.

In any case, I think absolutely no one would have used this anyway, so let us just go ahead with it 👍