The odd thing about Greg's decanonising romance was that he himself was largely responsible for all the Matau/Nokama stuff (or rather Matau --> Nokama stuff, given that she never reciprocated except platonically). The narration at one point says "Nokama was his friend, and he sometimes wished she could be more than that", and the Hordika arc has Matau heavily imply he wants her to be his mate, (or as close as biomechanical biology would allow them to get) so it's pretty hard to deny Greg's role in it.
So 2006 was the straw that broke the camel's back, either because Greg didn't like the idea of writing a couple that actually had mutual feelings as main characters (Hahli and Jaller), or having a Toa with a crush on a Matoran was too awkward for him, even though said Matoran wasn't around (poor Hewkii).
I think it actually wouldn't have changed all that much if Hahli and Jaller had remained romantically involved throughout the 2006-2007 arc. They both perform heroic acts to save each other's lives at various points (Jaller jumping over the volcano, Hahli wrestling a shark, etc), so those moments could be given more weight. Then there's the potential for tension in the relationship- Hahli was always kind and supportive towards Matoro, which would give Jaller yet another reason to be jealous of him (justified or not), and Jaller's attempted nova blast would give anyone trust issues. It's also established that although Jaller cares about Hahli, he's frequently serious and not good at supporting other people emotionally, leaving her frustrated and lonely. Meanwhile, Hahli's growing confidence leads to her frequently leaving the team behind and them wondering if she's betrayed them for the Barraki, which would be interesting to see more from Jaller's point of view. A breakup might be a possibility at that point, with a possible reconciliation arc later on. It's pretty clear that they need to talk to each other more, romantic connections or not.
Macku is only mentioned by name once in the arc (and by Teridax, at that) but her presence (or rather, absence) is felt. At one point Nuparu, acting as a stand-in for the audience, asks Hewkii whether he misses anything about his old life, and Hewkii's response feels a lot like "Yes, of course I miss my girlfriend! Shut up nerd, I don't wanna talk about feelings!"
As to what form romance might take in the MU (had it not been decanonised), I see it more as a strong emotional attachment or intellectual connection and wanting to spend time together without others rather than a physical attraction (which I agree doesn't make sense for them). But because it wasn't intentionally programmed in, they don't have a well-defined concept of romantic love, "marriage" is a political union, and "dating" is only a thing archaeologists do with old artefacts. Wanting to hang out with one specific person a lot, hold their hand and/or cuddle them was probably considered weirdly clingy by some, perfectly fine by others, and "mostly okay as long as it didn't distract them from work" by the Turaga.
That said, Greg did eventually decide that after being socially exposed to the idea of romance on Sphereus Magna, it might be theoretically possible that some biomechs would possibly maaaayyyybe consider thinking about adopting the custom themselves, but that it would take a long time, and certainly not for everyone. I am 200% up for this, because robots trying to adopt romantic customs (and occasionally failing) is the best.