doubleohfive, the legacy walls included sailing ships and the NASA space shuttle (no Richard Branson VSS Enterprise either ). No "in-universe" reason to exclude it.
Of the three legacy walls, one featured UFP starships exclusively (the one on the E-E). Of the other two, one omitted the nuclear carrier (the TMP one) while the other one omitted the war-hero conventional one (the E-D one - the TMP wall did actually have that one). Both omitted half a dozen assorted sailing ships, and no doubt also half a dozen spacecraft of all sorts. Archer's "poor man's version" only had four vessels in all, again omitting CV-6 but also all other spacecraft besides the shuttle test article.
It's quite difficult to see a pattern of completeness there... A starship or three could well slip through. Indeed, the TMP wall had the famous if mysterious ringship which clearly was an advanced spacecraft, but the E-D wall did not...
The Borg message sent in regeneration made little difference? Yet the Federation encoutnered the Borg (Neutral Zone and Q-Who) at exactly the time that was predicted.
The prediction itself made no sense, though. How could T'Pol figure out when the message would reach its destination? She only knew the direction and the speed. She couldn't know the distance (and thus the time) unless she knew the destination beforehand, and how could she know that?
In all the encounters with the Borg, the cyborgs or supposedly neutral observers have never suggested that any of those would be a first encounter between the Borg and the Earthlings. As for the Earthlings themselves, they thought they had a first in "Q Who?", but they were doubly mistaken - they quickly realized the connection to "TNZ" but never realized that the Hansens had already been there and done that. So it's possible there were no further contacts in addition to those shown, but it's rather probable that there were, quite possibly long before the Hansen case. (Certainly the Borg seem to have clashed with the Federation many times since, without the camera being anywhere around - dialogue in "Scorpion" and ST:FC makes this rather clear.)