We had a relatively small number of support requests in regards to this. Those that we did have came from quite dissatisfied customers, though. Most thought that their redirect isn't working and some of them believed that it hasn't been working for a long time.
The common issues were:
a) redirect owners or their visitors were manually entering a https address
b) site owners had a site before and later turned it into a redirect
In case of a)... well, it's a honest mistake and in case of visitors, the damage from thinking that the address doesn't exist might be real. It would simply be good practice to have this working these days, especially since certificates are basically free.
In case of b), when we're informed we set them up similarly as Peter described above. But when they do it themselves, they expect Plesk's redirect functionality to work with https out of the box. In general, these sites had encryption enabled and rightfully expect to have it working on the redirect because of the stale links and search engine entries.
It all boils down to the address itself. The encryption of the redirect isn't really needed, but customers rightfully expect the https address to work without issues.
I honestly think that Plesk should add this functionality ASAP and just be done with it.