I am referring to the "PSoC 4100S Plus PSoC4 Registers Technical Reference Manual (TRM)" document.
Is the content described here guaranteed?
Is it guaranteed that slave mode will be enabled if you write '1' on the 30th bit of the I2C Control Register(SCB0_I2C_CTRL) on page 1252?
*It is a condition that SCB0_I2C_CTRL  = '1' is written normally.
It is a condition that the write does not fail.
Yes. The content described in TRM is guaranteed by design and these registers are registers used in code(API) generated in PSoC Creator.
You can check detailed corresponding register addresses in cydevice_trm.h generated by each project in PSoC Creator.