Apple has released iOS 12.5.4 for older iPhone, iPad, and iPod touch devices.

For iPhone iOS :
12.5.4 (5s GSM): iPhone_4.0_64bit_12.5.4_16H50_Restore.ipsw
12.5.4 (5s GSM+CDMA): iPhone_4.0_64bit_12.5.4_16H50_Restore.ipsw
12.5.4 (6+): iPhone_5.5_12.5.4_16H50_Restore.ipsw
12.5.4 (6): iPhone_4.7_12.5.4_16H50_Restore.ipsw

For iPad OS :
12.5.4 (iPad Air Wi-Fi): iPad_64bit_12.5.4_16H50_Restore.ipsw
12.5.4 (iPad Air Cellular): iPad_64bit_12.5.4_16H50_Restore.ipsw
12.5.4 (iPad Mini 2 Wi-Fi): iPad_64bit_12.5.4_16H50_Restore.ipsw
12.5.4 (iPad Mini 2 Cellular): iPad_64bit_12.5.4_16H50_Restore.ipsw
12.5.4 (iPad Air China): iPad_64bit_12.5.4_16H50_Restore.ipsw
12.5.4 (iPad Mini 2 China): iPad_64bit_12.5.4_16H50_Restore.ipsw
12.5.4 (iPad Mini 3 Wi-Fi): iPad_64bit_TouchID_12.5.4_16H50_Restore.ipsw
12.5.4 (iPad Mini 3 Cellular): iPad_64bit_TouchID_12.5.4_16H50_Restore.ipsw
12.5.4 (iPad Mini 3 China): iPad_64bit_TouchID_12.5.4_16H50_Restore.ipsw

For iPod Touch :
12.5.4 (6G): iPodtouch_12.5.4_16H50_Restore.ipsw