The game would render characters in the Private Use Area if the appropriate font sheets were present.
Even when using a resource pack which assigns these glyphs an appearance, the game does not render them. Inserting these characters is not prevented; the game just fails to render them.
Steps to Reproduce:
1. Create a resource pack
2. Add font files such as "unicode_page_e0.png" or "unicode_page_f8.png" to assets/minecraft/textures/font in your resource pack. These may be duplicates of existing font files, if you wish.
3. Attempt to type these characters in-game, or paste them into a book
4. Observe that, regardless of how the characters appear in your resource pack's image files, the game instead renders blank zero-width characters.
Why this is likely a bug and not intentional behavior:
Note that these characters are no different than other Unicode characters and are well within Minecraft's range of supported unicode (specifically, U+E000..U+F8FF, where Minecraft supports U+0000..U+FFFF). Resource packs can assign appearances to almost all glyphs in the game, apart from these and "forbidden" glyphs.
When a character is "forbidden" in Minecraft (for example, various control characters), it cannot be typed or pasted into books. However, these characters can be typed and pasted; they just fail to render: they aren't "forbidden".
The purpose of the Private Use Area in Unicode is so third parties can add custom characters to their fonts - it would be well within these guidelines for a resource pack or Mojang themselves to add characters here.
If Mojang does not intend for these characters to be used in Minecraft, then they should not be typable in the game. As it stands, by allowing users to input them, but refusing to render them, these characters aren't exactly usable for any purposes aside from censor evasion.