Are there any examples of Star Trek TOS aliens being assimulated by the Borg in any of the following versions of the Star Trek TV/Movie franchise?
By the standards set forth in the question, there are 55 original series species that are "encountered/discovered" during TOS Star Trek. Of those 55 species, only the Romulans meet the criteria of
- having been discovered or encountered during "first contact" during the TOS Star Trek.
- having had members of their species assimilated by the Borg.
I put quotations around first contact since the Federation had been at War with the Romulans before TOS but had not communicated (beyond trying to blow up their ships) before the 1966 episode of TOS called "Balance of Terror" first aired on September 13, 1966 and was the 14th episode of Star Trek to be aired.
The Romulans are identified by the Borg as Species 3783.
I'm not sure if you are willing to include games in the franchise, here's one example
In the Star Trek Game Star Trek: Legacy, the Borg are featured midway through the TOS era as "Assimilated Klingon ships". In the final mission, once the player has completed the primary objective, a confrontation with T'urell ensues; she departs, and the player must destroy a Borg Sphere.
Also, same Wiki article has 2 more book examples, one of a race whose creation is mentioned in TOS, and one, of an attempt to assimilate TOS's Spock:
The Peter David novel Vendetta reveals that the planet killer weapon from the Original Series episode "The Doomsday Machine" is a prototype for a weapon against the Borg, with a woman whose race were destroyed by the Borg trying to use the weapon against them despite the damage she will cause on her journey due to its need to consume planets. David revisited this concept in a 2007 sequel novel, Before Dishonor, which features the Enterprise-E working with Spock and Seven of Nine to reactive the original planet killer to stop the Borg.
In William Shatner's novel The Return, Spock is nearly assimilated by the Borg, but is saved because he mind-melded with V'ger, an earlier form of the Borg. Using the information he subconsciously acquired in the meld, Spock is able to lead a crew of Enterprise officers- consisting of the Enterprise-D senior staff, himself, Admiral McCoy, and the resurrected Kirk- in a Defiant-class ship to destroy the Borg central node, severing all branches of the collective from each other and limiting their future threat.
We saw a couple of Klingon Borg in at least voyager. Klingons are definitely TOS aliens.