While you might be looking for a specific download for a "fixed space" (monospaced) font related to iOS 15.4, it’s important to clarify that for use in the main interface or menus in the same way you would on a PC or Mac.

Improved spacing for symbols and punctuation.

font ensures that every character (from a wide 'M' to a narrow 'i') occupies the exact same amount of horizontal space. Creative Market

: This is the default fixed-width font for Apple platforms, designed for high legibility in code and technical contexts. monospaced()

The cursor blinked. Steady. Monospaced. Perfect.

This yields a font with perfect fixed spacing and every possible coding glyph.

iOS 15.4 has a bug with certain .otf monospaced fonts. Convert your font to .ttf (TrueType) using a free online converter. TrueType renders significantly sharper in Safari Web Inspector and Terminal emulators.