How to search only current directory in Nautilus (restore old behaviour)

In short, you can't! Not currently with Debian anyway, the dconf key has been removed.

It's been marked as a bug for the last two years, so it doesn't look like anything is going to change in any hurry.

Best option is to switch to nemo, branched from gnome2 Nautilus, still maintained but without the insane new "features" they've added to Nautilus itself.

I think this might achieve what you want.

Files->Preferences->Search&Preview:
Search -> Search in subfolders: -> Never

It seems that this setting would cause the search in the current folder only.

Or you might change this setting in CLI:

gsettings set org.gnome.nautilus.preferences recursive-search 'never'

BTW, My debian release is stretch.