Yours is an interesting point, but I’m not wholly sure we want this fixed.
The technical explanation for what I believe is happening is probably going to either confuse or bore the pants out of most people reading this, but here goes anyway.
I’m pretty sure that Stack Exchange searches are done using something called the Unicode Collation Algorithm at level-one equivalence, also called the primary level or strength.
Primary strength here corresponds to “alphabetical” ordering only. That means that case and diacritics don’t count, just the "basic" letters themselves, and indeed many code points wind up being equivalent there.
In particular, at the UCA-1 level, all these count as the same letter:
U+0044 D LATIN CAPITAL LETTER D
U+0064 d LATIN SMALL LETTER D
U+00D0 Ð LATIN CAPITAL LETTER ETH
U+00F0 ð LATIN SMALL LETTER ETH
U+010E Ď LATIN CAPITAL LETTER D WITH CARON
U+010F ď LATIN SMALL LETTER D WITH CARON
U+0110 Đ LATIN CAPITAL LETTER D WITH STROKE
U+0111 đ LATIN SMALL LETTER D WITH STROKE
U+0369 ○ͩ COMBINING LATIN SMALL LETTER D
U+1D30 ᴰ MODIFIER LETTER CAPITAL D
U+1D48 ᵈ MODIFIER LETTER SMALL D
U+1D9E ᶞ MODIFIER LETTER SMALL ETH
U+1DD8 ○ᷘ COMBINING LATIN SMALL LETTER INSULAR D
U+1DD9 ○ᷙ COMBINING LATIN SMALL LETTER ETH
U+1E0A Ḋ LATIN CAPITAL LETTER D WITH DOT ABOVE
U+1E0B ḋ LATIN SMALL LETTER D WITH DOT ABOVE
U+1E0C Ḍ LATIN CAPITAL LETTER D WITH DOT BELOW
U+1E0D ḍ LATIN SMALL LETTER D WITH DOT BELOW
U+1E0E Ḏ LATIN CAPITAL LETTER D WITH LINE BELOW
U+1E0F ḏ LATIN SMALL LETTER D WITH LINE BELOW
U+1E10 Ḑ LATIN CAPITAL LETTER D WITH CEDILLA
U+1E11 ḑ LATIN SMALL LETTER D WITH CEDILLA
U+1E12 Ḓ LATIN CAPITAL LETTER D WITH CIRCUMFLEX BELOW
U+1E13 ḓ LATIN SMALL LETTER D WITH CIRCUMFLEX BELOW
U+2145 ⅅ DOUBLE-STRUCK ITALIC CAPITAL D
U+2146 ⅆ DOUBLE-STRUCK ITALIC SMALL D
U+216E Ⅾ ROMAN NUMERAL FIVE HUNDRED
U+217E ⅾ SMALL ROMAN NUMERAL FIVE HUNDRED
U+24B9 Ⓓ CIRCLED LATIN CAPITAL LETTER D
U+24D3 ⓓ CIRCLED LATIN SMALL LETTER D
U+A779 Ꝺ LATIN CAPITAL LETTER INSULAR D
U+A77A ꝺ LATIN SMALL LETTER INSULAR D
U+FF24 Ｄ FULLWIDTH LATIN CAPITAL LETTER D
U+FF44 ｄ FULLWIDTH LATIN SMALL LETTER D
There may be a way to search exact code points using some sort of quoting trick, but unless such exists, I doubt you’d be able to get SE to change how searching works just for us. I believe they use something called Elasticsearch, tuned for English only.