const StringRef text

Floats can be passed by value

These aren't return variables, since they're local functions. Suggest text_size and text_free

Looking at this code path, it looks like the curve data is generated for every single character here, only to be created later again when you call BKE_vfont_build_char

The API is pretty awful, but I wonder if one of the options like FO_DUPLI calculate only the character positions to avoid the extra work here.

Generally it's preferable to use StringRef instead of a const reference to a string. Though I won't speak for the strings of non-char types elsewhere in this file.

add_overwrite seems wrong here, since you already tested with contains above. We should really avoid generating the same character twice

I don't think this is true, this node creates an instances component from scratch.

Right, that would wait until we support attributes on the instances component, which shouldn't be too hard.

These enums can be defined directly in def_geo_string_to_curve.
I think it's nice to decrease the scope like that, even if it makes the function longer.

The final periods are added to descriptions automatically and don't need to be added here.

The pivot point property doesn't do anything in the patch now, and should be removed.

I would suggest returning a struct like this rather than changing the input parameters:

struct TextLayout {
  Vector<float3> positions;

  /* The part of the input curve that didn't fit in the text box. Only set in truncate mode. */
  std::optional<StringRef> truncated_text;

  /* This should have a comment about what it means. */
  float size;
};

This way it's more clear what is an input and what is an output.

const

If you're just freeing the r_text after the function, do you have to give it as an argument? Or could you just pass null?

Add a comment about the FO_DUPLI argument, and the reason for that. (Did you verify that it does what I thought?)

Here you can reserve text_size size in the vector to avoid reallocation when it grows.

What's the runtime of std::string::replace? I wonder if it would be more efficient to build a separate string, since this might require moving the characters after the replacement?

Add a comment about the meaning of the map this function returns.

Is calling this necessary? I'm not sure what the purpose would be.

It seems simpler to avoid assigning a temporary value here.

Zero size is what the vector gets by default, no need to do that explicitly.

Pivot points are unused, the related functions and variables should be removed for now.

Always use braces: https://wiki.blender.org/wiki/Style_Guide/C_Cpp#Braces

Span is small (16 bytes) and should be passed by value, otherwise it adds another pointer indirection to element access (if not optimized away).