Not relevant to breed standards but---
Q: Why does anyone dock tails?
The usual answer is something to do with maggots/daggs
The real reason is probably fashion & tradition
I would argue that
1) I would want a long tail to swish away flies
2) Dirty tails are more due to faeces texture and wool type than tail length---a clean /bare/short woolled tail with selection for daggs would give a simple and less painful genetic solution?
(Tail length, dagginess are both very heritable traits)
I would wholeheartedly agree that we should all be breeding for traits which mitigate against fly strike. Which in my experience (of small numbers of a fair few breeds and huge numbers of a small handful of breeds) would mean using a lot of Manx in our breeding programmes (although the only strike I have ever had in a Manx or Manx cross was on a horn bed, so maybe we would also have to go polled, or select for smoother horns and select out some of the specific shapes which are more likely to get damaged and so attract flies.)
My own breeding standards are to not breed from any ewe which has had fly strike or which require dagging when others do not - unless it's Quincy my black Wensleydale

. She and her descendants do cause us 85% of our work - but their fleeces and personalities are worth it! However, if I was running a bigger flock and/or could not rely on experienced, capable shepherding, I would have to remove all those lines from the flock.
It is our Wenseys and descendants alone which we now have to dock; every other sheep in the flock has the northern shorttail "fluke-shaped" tail. In addition, most of those with a lot of Shetland in the mix self-shear around the tail and rump early on in the season.
I left one of Quincy's lambs long-tailed, and I will never stop regretting it. He had the softest fleece of any of her offspring, but that long woolly tail and the line's propensity to need dagging several times a year were a bad combination. I caught the strike before any real harm was done, and in truth I had had "dag Bumper" on my To Do list for a few days when it happened so I could have avoided it, but because I could not rule out it happening again and with a worse outcome, I sent him off and have docked any long-tailed Quincy descendants since.
An alternative to docking maybe could be to crutch those sheep very close early on, before the flies start. But I am not a huge fan of preventative crutching, because if they do then get a mucky bum, you haven't enough wool growth to cut through to clean them up.
Lots of other factors can help keep bums clean... Stay on top of worms; give mineral drenches; manage grazing so that those with a propensity to mucky bums are not on lush, fresh grazing; etc. But you can never manage your way out of all dirty bums, you can only nearly achieve that through breeding.