Android - Can an Android phone be used as a Bluetooth headset/speakers?

As of right now the bluetooth API only supports pairing with the android device as the master.

Here's a XDA article by Mishaal Rahman that basically says:

Even without A2DP profile on Android (which is not implemented even in Android 8 yet), you can do a low-quality Bluetooth streaming via Hands-free Profile (HFP) to Android from another Android phone (sorry iPhone owners!)

Install Tasker and a AutoVoice plugin on the source phone, and create the following Tasker profile:

Profile: Reroute Audio (192)
State: BT Connected [ Name:BMW 15384 Address:* ]
Enter: Anon (199)
A1: AutoVoice Ctrl BT [ Configuration:Enabling Headset Sound Timeout (Seconds):0 ] 

Exit: Anon (204)
A1: AutoVoice Ctrl BT [ Configuration:Disabling Headset Sound Timeout (Seconds):0 ]

Warning: I have not tested this myself.

Warning: quotation about HFP audio quality:

Note: because the Hands-Free Profile is not meant to send high-fidelity audio streams, the audio quality of your music will suffer. This is just the nature of the Hands-Free Profile and there’s nothing you can do about it without either upgrading your stereo’s Bluetooth to bring A2DP support or to stream through another peripheral.