Tapering is something that is highly specific to each athlete, so getting it right takes a bit of experimentation for each person. In college, we were told a reduction in volume of 60-90% 5-21 days in advance of the event. The key with that was to keep the intensity of the work outs, just reducing the volume. For instance, if you typically had been doing at 1.5 hour week day ride, with 2x20 min hard, than in taper week (or weeks) that would look like a 36 min ride, with 1x 16 min hard. That all being said, if i reduced training volume by 90% 3 weeks out, i would race like crap.
Now... with that said, it is totally dependent on a tun of factors, and this is probably why you have not seen a "taper plan for 70.3)
The first thing to consider is what your weeks before the taper period looked like. For your half ironman, how many hours a week have you been averaging in the months leading up to it? If someone is "only" putting in 6 hours a week (which I have seen for 70.3) am I going to than reduce their volume by 60%? Probably not. and Definitely not for 21 days. However, I have athletes who are doing 20 or more hours a week on average, than yeah, Ill be reducing their hours pretty significantly, while keeping intensity. Often adding intensity. The goal would be to get them to the start line fresh and recovered, and by keeping the intensity, their fitness will stay sharp, and wont go down by too much, because we have kept the systems engaged.
Now, ill give you what my taper looks like for my final 70.3 of the season, and remember, this is all based on what you have been doing ahead of the season.
3 weeks out - Overload week, 19.75 hours, with some serious intensity.
2 weeks out - 13.5 hours, short bursts of intensity, high and hard.
1 week out (race week) - 5.5 hours, recovery work outs, with some short sprints, and race pace efforts. But short.
So for your, what do your weeks look like leading up to the event?
12-14 hours a week for me, 2 run days are spring intervals for 1 hour, and 2 bike days are also sprint intervals. I'd say I go pretty deep in the well on those sprint days.
Anyways, I think my taper plan doesn't suck, but with a small change to reduce intensity on that final week just a hair.
Doing nothing is not an option.