More generally, I really don't see the point of paying for custom-fitted clothing without first confirming that the clothes are going to fit.
a neat alternative would be, imo, to have them keep your old measurements, but store them online where you can see/edit/update. old ones prompt for re-measurements
Opt-in would be fine, but I have to say as someone who is in the process of aging, I'm not the same shape I was 5 years ago. 6 months probably, but I doubt for many people the data is actually usable without a refit after, say, 3-4 years.
It doesn't seem that much of their variable cost would get shaved off by already having one's measurements, which makes me think this wouldn't help them with a lowest-price strategy. Maybe a higher-end market focus.
But as other have already said: body size data can be expected to expire often. I'm an edge case, but my weight has oscillated between 195 and 165 a few times in the last year.