Apparently, one can still register single letter domains in .bike, .ventures, .holdings – but some come at a premium registration and renewal price, as revealed via a comment at Elliot’s blog DomainInvesting.
The following single letter dot .Single domains can be registered right now:
*** d.singles $99
*** n.singles $99
*** o.singles $99
*** p.singles $99
*** r.singles $99
*** s.singles $99
*** u.singles $99
*** v.singles $99
*** x.singles $1,600
*** y.singles $99
*** z.singles $99