how real is the danger of an "S" trap actually siphoning so that sewer gas will be admitted?
Like NH master says,
If you have it, you can leave it.
If it's new, then it should be done right.
There are limits on how long tail pieces can be, the longer they are, the more likely that water dropping down will siphon a trap.
I did one deep tub install where the horizontal trap arm was quite a bit lower in the crawl space.
It wasn't my original plumbing, just going in to hook up a tub.
When I drained the tub, I noticed that the trap was siphoning.
Uh, not good.
The distance does make a difference.
And that's why we now longer put p-traps under the floor.
We know better now.
You don't really have to think these things out that much,
just do it and you'll know why the codes changed.
I've had a few houses where I've done some funky stuff, knowing it was going to be torn down anyway, just to try some ideas out.
I'm a curious guy.