Are the following unicode points used for anything? It seems like they would have been combined with other letters to create ŧ, ł, ø, etc. in a font that doesn’t support these glyphs. Would adding them to a font today serve a purpose?
u+0035 Short Stroke
u+0036 Long Stroke
u+0037 Short Slash
u+0038 Long Slash
And the same for U+0312, Combining turned comma above. Is that just for creating ģ?
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Re. U+0312. It's useful, as you note, for creating the ģ as a composite, and since the shape exists, it might as well be encoded and put in an appropriate mark position class. There's always the chance that some orthography, somewhere in the world, today or tomorrow, will use it as a distinct diacritic mark.
The others —0335, 0336, and 0337— aren't present in Unicode as components. But what John said about 0312 also applies to them. All are useful for creating composites like Ł, Ƀ, ѣ, ꙃ, ᴃ or Ꝅ.
If you plan a really deep language support, there are some non-encoded characters which also use these diacritics, as barred Iota or L with double stroke.
Maybe the first three have additional importance for non-coded phonetic composites. They are present in Stone Serif Phonetic and SIL fonts.
\not\subset
), and TeX users come to expect they can negate any symbol, so some TeX engines supporting Unicode and OpenType has special “math accent” for overlays that centers them vertically and horizontally relative to their bases.