Learn English – Are “preaching to the choir” and “preaching to the converted” synonymous

As this chart shows, both terms have become far more common over recent decades...

My feeling is that encoded is associated with technical contexts where the method of encryption is more important than the encrypted information itself. Thus, from Google Books...

sent a coded message:4060 hits; sent an encoded message:62 hits.
(The human) genome is encoded (in DNA):311 hits; ...genome is coded...:49 hits.

In more metaphorical contexts ("She declined my offer of a drink at the bar, which I took as a coded message that she didn't fancy me"), I definitely wouldn't use encoded.

Normally with these sort of things, I like to hit a lot of different dictionaries. Here though, the definitions you give tell it all:

very confident and direct about saying what you think, in a way that is not socially suitable

(of a person) bold or overfamiliar in manner: I am not usually a forward sort of person

(Emphases mine)

In the definitions you give, the fact that forward contains an excess that makes it unwelcome is given clearly of the definition. It's also implied in the example, as it wants to deny the quality - which we would of a negative excess, but not of being honest and direct.

The classic example of a straightforward person, is the sort of characters James Stewart used to play; politically open rather than manipulative, not given to intrigue, and likely to say "well, 'cos I think you're a fine looking woman so I'd be mighty pleased if you'd accompany me" rather than subtly seductive.

That last bit is the closest those characters to being forward, but they'd be horrified and apologetic if it was suggested that they were. A forward person shares the bluntness of the straightforward person, but will continue with advances to a degree that seems creepy, or where the a straightforward person would be honest about personal matters, the forward person would be honest about personal matters that have no bearing on the conversation and at a time when it is not appropriate to bring them up.

So while the forward person is indeed honest and direct, this unpleasant excess is a core part of the definition. Since honesty and directness are generally considered positive qualities, it's not reasonable to consider it a valid Jeopardy answer for that definition.