Ok, this request has been added to the tracker:
https://winscp.net/tracker/919

Desktop shortcuts is a good solution, but not very convenient.

martin wrote:

You can make a location profile for that.
https://winscp.net/eng/docs/ui_locationprofile

Location profiles works only in Commander interface. It has no problem: on opening synchronize dialog the current local folder already selected.
But I use "Explorer" interface, a feature offered only for him.

You can make a location profile for that.
https://winscp.net/eng/docs/ui_locationprofile
Or even desktop shortcut
https://winscp.net/eng/docs/commandline#operations

When synchronize several subfolders of sites – everytime we need to select appropriate path in the "Local directory" history.
The same with the "Keep remote directory up to date".

Why not to save last selected "Local directory" relative to the current remote path...