Do you have an example of that?
I don't believe bar-lined 7 is an actual font set glyph. By hand you should always write the 7 with a bar since it distinguishes it from the 1, especially for people like me that have illegible writing.
In HTML, it's simply a matter of:
Which gives the same effect.
If you can find the actual page it was used on though, then we can take a look at the ascii character in use or whatever entity was used. "7" is pretty obvious in typed text, so I don't think it was ever added as a character (but potentially a font set uses it).