We do not have any date set for 5.8 release.

All security fixes from PuTTY 0.64 were included in 5.7.1 already.
See https://winscp.net/eng/docs/history_old#5.7.1

Bottomline: I am wondering when version 5.8 will be released. I have been monitoring the history page for more than a month now but there are just new features added to the "not released yet" versions in contrast to actually releasing them (at least during the period of time I monitored that page).
I would like to see this happen rather sooner than later because of the speed improvements but first and foremost because of the "SSH core upgraded to PuTTY 0.64" since an important security fix was implemented in that version.