in a text object I tried to insert some unicode codepoints from the Emoji section (e.g. this one: https://emojipedia.org/grinning-face/, codepoint=😀 U+1F600). I used the font Segoe UI Symbol (I'm on windows 10) for the text object. The grinning face is for sure in that font, but in blender there is always a rectangle displayed!
I tired the copy and paste method and a scripting method with bpy.data.objects["demo"].data.body = "\U0001F600" (my textobject ist named "demo", but bothe results to the same rectangle.
My guessing is that Blender is not able to process unicode codepoints above hex FFFF, but this is only guessing