Does Android send Wi-Fi probe requests to saved hidden networks? Let's put this scenario:
I manually add an existing Wi-Fi network that does not broadcast SSID. For some reason the encryption passphrase doesn't match, so authentication is impossible. Previously hidden SSID is now shown in Wi-Fi networks list.

Click on hidden SSID that is now shown and forget its settings. Click on the same hidden SSID now presented on the list and set a correct passphrase. Authentication is done successfully.

My question is if the network is still considered a hidden network on wpa_supplicant.conf?

In the future if Android client does not send a direct probe request for a specific hidden SSID and instead sends a broadcast probe, will it be unable to detect the network?
A hidden or non-broadcast WiFi Access Point remains always hidden i.e. it always sets SSID to null in beacon frames it sends. However once you manually add its SSID to a supplicant, it depends on the latter whether it explicitly scans the former in future or not. Android's WiFi client is wpa_supplicant, the same is most commonly used on other Linux-based OSes. From wpa_supplicnat configuration manpage:


  scan_ssid
  SSID scan technique; 0 (default) or 1.  Technique 0 scans for the SSID using a broadcast Probe Request frame while 1 uses a directed Probe Request frame.  Access points that cloak themselves by not broadcasting their SSID require technique 1


So scan_ssid=1 is usually the default behavior on most OSes. Also Windows built-in supplicant can "connect even if the network is not broadcasting".


  In the future if Android client does not send a direct probe request for a specific hidden SSID and instead sends a broadcast probe, will it be unable to detect the network?


Yes definitely. But scanning hidden networks is default setting up to Android 8. In Android 9 a new configuration was added to allow explicitly setting the behavior:




  My question is if the network is still considered a hidden network on wpa_supplicant.conf


In Android whole configuration is not added to wpa_supplicant.conf file but retained by Android framework. Since Android 8 it's stored in WifiConfigStore.xml files located under /data/misc* directories. According to user selection, scan_ssid=[0|1] - along with other configurations - is passed to wpa_supplicant process over HIDL - an Android specific IPC - or possibly over global ctrl_interface.

To summarize, yes Android (by default or optionally) sends direct probe requests to saved non-broadcast WiFi networks.