Braille pattern dots-1245678

A braille cell with seven of its eight dots raised. An 8-dot cell has two columns of four: dots 1, 2, 3 and 7 down the left, 4, 5, 6 and 8 down the right. ⣻ fills all of them except dot 3, the third dot on the left. Unicode gives every one of the 256 possible arrangements its own character, so this is a shape, not a letter.

Braille itself is the raised-dot writing that blind people read by touch. Louis Braille, blind from childhood, met Charles Barbier's raised-point system at his Paris school in 1821, reshaped it into a six-dot cell by 1824 and published it in 1829. The two extra bottom dots came later, mainly in computer braille, where eight dots let one cell stand for any character on a keyboard. Which letter or sign ⣻ stands for is set by the braille table in use, and that differs between languages and codes.

Why does it turn up in posts and usernames?

Mostly as a pixel in text art, not as braille. Because the 256 cells cover every on/off combination of a 2×4 grid, people use them to draw pictures, faces and memes out of plain text in chats, comments and bios. A nearly full cell like ⣻ works as a dark shade with a small notch. In that use nobody means to write braille, and a screen reader will announce the art as a long string of dot numbers, which is tiring for blind users to hear.