The easiest way to parse this is that there are Hackable things and things that can be Hacked. There's significant overlap. I'm with @Mahtamori in that it's the HSN3 Holoprojector where the problem is: it uses Hackable to mean 'ability to be hacked'.
Well I'm glad that thread got linked because I didn't know that I had to immediately tell my opponent, or vice versa, whether something holo'd is able to be hacked and furthermore what programs work on it. Although it occurs to me I can't remember it coming up, or making a difference. I do think it makes sense though, could be clearer mind you. But I do have a question; if I drop a repeater next to a HI and it is in fact a KotHS pretending to be another kind of HI... is my opponent required to divulge that the HI is hackable and vulnerable to X Y and Z programs? Because although I might presume that the "fake" HI is hackable, when the Repeater goes down, that Private Info becauses Open, they must divulge it. Does it count as being already divulged because they're pretending to be something with the same signature?
You certainly must tell your opponent that the HI is Hackable. How you do this is basically up to you: courtesy list and pointing out that it's model 'x' on the list is probably the best practice. Ultimately I wouldn't expect you to tell me again information that hasn't changed. I know of no requirement in the rules that would require it, but you always can opt to. Opinions vary on whether you need to explain which programs can target it. This has not been definitively resolved. Say you had it Holo'd as a Father Knight AHD: depending on your interpretation of the rules you'd either inform your opponent that Redrum will not work against it or you'd just say 'this trooper is Hackable'. My opinion is that the rules were intended to mean that the quantronic characteristics of a Trooper are not obscured by Holoprojectors and consequently a Hacker is entitled to know what programs will function against that Target.
While Spotlight works on any enemy within range even if it lack Hackable Characteristic; what do you have to say about Hack Transport Aircraft ? It simply works against AD:Combat Jump, it doesn't say if the MI troop need Hackable Charateristic or not, and doesn't say if the AD troop gains the Hackable Characteristic. And in Oblivion, they made sure to include "Hacker" distinctly than "the Hackable Characteristic (examples in bracket...)"; which would be unnecessary if Hacker automatically grants the green icon they could have added it inside the example brackets. The rule about how you get the icon, as other rule, is clear in that it must tell you how otherwise you don't, and HD doesn't tell you to gain that Characteristic. Therefore the only way to have it, is to have it printed on your troop profile. In the case of some trooper where HD is not one of several loadout option, it doesn't have the icon. So the response that the icon is not shown (but should be considered to be auto-added despite lack of any rule text) for models who can optionally be hacker doesn't hold. "The target of this Hacking Program need not have the Hackable Characteristic." this line found in several program is more a reminder, as with or without it you still need to fulfill all criteria to be targeted and there is no written criteria (in HD rule or elsewhere) that programs "never work against models who lack Hackable Characteritic unless otherwise specified". So that line is lifting a restriction that does not even exist.
Slightly different topic, but can a hacker still be hacked after suffering the effects of blackout or E/M ammo that switches off hacking devices ?
Yes. The requirement is that they're a Hacker. Any Trooper with a Hacking Device is a Hacker. It's irrelevant whether the device is functional. This is also important for Objectives and some classifieds.
RAW you kinda do not have to tell your enemy anything other than hacking characteristic. They should update/clarify this. Because especially the Kanren leads to alot of bad blood between players. It makes a very bad atmosphere if one of players is accusing the other of cheating.
Well, first of all, Hack Transport Aircraft is actually weirder than that, because you're "hacking" the order of declaring AD: Combat Jump, which results in a FtF against the AD trooper, so he isn't actually hacking the trooper. Stil, I have to concede your second point because I can't see anything that can argue against it.
I know this is anecdotal but he tends to almost always hide as a Crane/Hsien with Martial Arts anyway, which means he have Stealth so most of the time you just walk past people anyways. I know that doesn't solve all problems but it's just that it mostly never comes up in my games at least (which might have something to do with the fact that I almost always use him as the Mad Trap profile, pretending to be a Pheasant Mad Traps).
I often (but not always) hide my Kanren as the wrong hacker, which makes it fun when a TAG thinks it can waltz past a "KHD" Kanren or when an opponent wastes tonnes of orders avoiding an actual KHD Kanren with their TAG/REMs. I think we can conclude when reading between the lines that what Holoprojector rules mean is that you need to reveal whether the Holoecho is targetable by hacking programs or not. It does seem like Holoprojector rule doesn't want you to have to reveal whether the Holo trooper is specifically a hacker or not (why else change wording so much from Hacking Device' rule?) Confirmation on this (do we reveal targetable by hacking, strictly Hackable characteristic or specifically what hacking programs are valid?) is needed officially, however.