Is there 'byte' data type in C++?

Yes, there is std::byte (defined in <cstddef>).

C++ 17 introduced it.

No, there is no type called "byte" in C++. What you want instead is unsigned char (or, if you need exactly 8 bits, uint8_t from <cstdint>, since C++11). Note that char is not necessarily an accurate alternative, as it means signed char on some compilers and unsigned char on others.