If you compare the sizes of the Low, Mid and High-tech rosters, the low-tech catalogue actually has a lot of ships. Granted it includes a large number of logistics vessels, but actually I think the midline roster has even more. And high-tech has a much slimmer roster, only two destroyers and two cruisers (not counting phase ships), but with three capitals and the heaviest focus on frigates.
So, it's not that low-tech is being left behind, but rather midline and high-tech are being better fleshed-out to match it.
As far as demonstrating new mechanics against low-tech vessels (ie. Onslaught), that's probably because they don't move much so are easy to hit with things. Also because of their reliance on armour for defence, their AI is happy to drop shields and tank hits on the armour, which I presume is good for demonstrative purposes as it gives a much more impactful visual effect, as you see physical damage being dealt.
(It's also worth noting that the last few major updates we've often seen strong additions for low-tech, with things like the Shepherd, Legion, and Atlas Mk.II, so if anything a break is warranted.)