What is the diference between Secure Simple Pairing and LE Legacy Pairing in BLE? My assumption is, that the SSP is the older one, rather not used today, am I right with it? Is SSP still used in the devices or it is rather state-of-art method of pairing.
That UG103.14 document seems to be written by some people at Silabs; it is not an official document written by Bluetooth SIG. That BLE would use Secure Simple Pairing is just wrong.
In Bluetooth Classic we have Legacy Pairing, Secure Simple Pairing and Secure Connections (the newest and safest one).
In BLE we have LE Legacy Pairing and LE Secure Connections.
You can read the following in the Bluetooth Core Specification 5.3, Vol 1 Part A (Architecture) section 5.4.1:
Bluetooth LE uses four association models referred to as Just Works, Numeric Comparison, Out of Band and Passkey Entry. LE legacy pairing does not have an equivalent of Numeric Comparison. In LE legacy pairing, each of these association models is similar to BR/EDR Secure Simple Pairing with the following exceptions.
• Just Works and Passkey Entry do not provide any passive eavesdropping protection. This is because Secure Simple Pairing uses Elliptic Curve Diffie-Hellman and LE legacy pairing does not. In LE Secure Connections pairing, the four association models are functionally equivalent to BR/EDR Secure Connections.
If you want to know more, I suggest you to read the whole of chapter 5 Security Overview. It's just 11 pages.