I find your question difficult to answer, because you mix up two things: consistency and overall quality.
"There seems to have been a consistent trend across many distilleries towards a poorer product over the last 2 decades. A case in point is Laphroaig 10yo. In the late 90s I was a regular drinker of this; recent bottlings are absolutely unrecognisable."
Rhetorical Question: Does a (in your opinion) different tasting Laphroaig make a poorer product?
When we talk about mere quality, there were huge positive steps in the scotch whisky industry over the past 20 to 30 years.
- The wash (beer that is destilled)
In the past, the washbacks, often made of wood, were cleaned by hand. This lead to a fluctuating cleaning result, and especially in warm weather periods (which even occur in scotland), it happend for the wash to be infected by vinegar bacteria, which made the resulting New Make taste sour. Today a lot of washbacks are made of steel and even the remaining wooden ones are washed by automated cleaning systems for a much better and more consistent result.
In the past the pot stills where directly heated with coals. When you have a high temperature concentrated on a small point, solid parts get burnt on pot still and lead to bitter notes in the New Make. Today a rotating rummager is widely used, which heats much more evenly and prevents this effect.
In the past the cask management was lacking details, or history, of a certain cask. How often was it used? Which destillery did it came from? This lead to casks being used to often and it made it difficult for the blend master to get a consistent batch for a certain bottling, since he has a recipie, but the casks where not labelled sufficient enough. Today a cask gets a barcode which contains all the information what a cask went through which makes trading easier and get more precise results over time.
One problem the scotch whisky industry is facing, is the huge growth in demand for Single Malt whisky. The the past, about 10% to 20% of the whisky production of a distillery went into Single Malts, the rest into the blending idustry. Today it is about 50% to 100%. This changes the cask selection for blends, since some destilleries don't provide them with casks anymore, and also for Single Malts. Because of the high demand, a lot of popular distilleries went out of longer matured casks and had to change their portfolio to younger bottlings, or create No Age Statement bottlings.
This had, of course, a huge impact on consistency in terms of taste.
So, to answer your question(s):
"Which single malt Scotches have improved over the last 20 years?": pretty much all
"Are there any single malts where there is a consensus that they have remained reasonably consistent": probably none