Samsung did not say why it removed the ability to set custom ringtones for each thread in the stock Messages app.

Troubleshooting text notification settings issues on your Galaxy Z Fold 4 Sometimes when setting a different notification sound for a particular contact you will experience issues.

From within that screen, tap Phone ringtone. In the resulting window (Figure A), tap the On/Off slider for Set ringtone for each SIM card (this will only appear if you have two SIM cards inserted).