There aren't any string concatenation OPs in bitcoin simply because there is no use cases for it in a payment system.
There used to be a couple of disabled OP codes in early version that are completely removed now that use to deal with string manipulation: OP_CAT, OP_SUBSTR, OP_LEFT, OP_RIGHT
There absolutely are use cases! Unfortunately the original construction could be abused to cause nodes to require petabytes of ram ... Fixing that requires imposing additional restrictions, and without planning for specific uses it can be hard to be confident that the restrictions don't break them.
... and demand for fancy use cases seems pretty limited-- which doesn't motivate people to do the design and validation work needed to reintroduce the operations.
), but I still can’t understand it thoroughly. Can you help me? Like information that is easier to understand. Especially about Many previously disabled opcodes are redefined to be OP_SUCCESS opcodes that unconditionally render the entire script valid to simplify soft fork upgrades. Can you explain it to me in detail? I don't quite understand what this upgrade brings about the OP_SUCCESS opcode?