If you are looking for a very precise answer, you will need to define very precisely what "least significant bit format" means. As evident in the comments, there can be a lot of interpretations of what that means, and a lot of details can be glossed over by a not very careful reader.
Example #1 - BMP image format, for black-white (1bpp)
For one example of "least significant bit first format", consider the BMP image format, 1bpp.
A black and white (bitonal, or 1-bit-per-pixel) image stored as BMP will have its row-zero, column-zero pixel stored at the bit 7 of the byte 0 of the pixel data area.
As a matter of arithmetic, the term "bit 7" always unambiguously refer to the bit mask
0x80, because it is the "7th digit of its binary representation". (the 0th digit is the bit mask
Example #2 - TIFF Fax
The original TIFF Fax specification was designed with hardware (electronic) implementations in mind, and therefore has to deal with electrical serialization and deserialization. With it comes the choice of which bit to send down the wire first. Thus, the specification allows for both, and therefore every TIFF image reader has to support reading both kinds of files and to perform the necessary conversion before decoding the bit stream.