At time of writing, w.wiki/b redirects to the Hungarian Wikipedia article for Budapest, w.wiki/m redirects to the English Wikipedia article for Mexico City, and w.wiki/q redirects to the English Wikipedia article "Queer" (in the LGBT+ sense). This strikes me as odd, because I would have thought it would be much more appropriate for those to redirect to Wikibooks, Meta-Wiki and Wikiquote respectively to match the interwiki table. (Wikispecies and the Media-Wiki wiki also seem to have been left out.)
Since the extension is still yet to be activated for everyone, I think it would be appropriate to prune the single-character URLs on or before 11 April so that it remains possible for the WMF to use most of them for new projects and initiatives in future without breaking a lot of incoming links, especially since it's (vaguely) possible that the short URLs could become significant in WMF branding/advertising.
w.wiki/a redirecting to Alan Turing's enwiki article is nice, but if the URL shortener is supposed to last a long time (i.e. on the scale of multiple decades), then it might be appropriate to change it to something else in case it would be preferable to use w.wiki/a for something else (most likely a new project with a name containing the letter "a"). There would be no harm in reserving a two-letter combination for Turing's article.