スラッシュドット・ジャパン

少なくともThe Unicode Standardでは、「使うな」というほど強いことは言っていないようですが。

長いけれど、The Unicode Standard 5.0.0の Chapter 2 [unicode.org]

2.3 Compatibility Characters
Compatibility Variants

Conceptually, compatibility characters are those that would not have been encoded except for compatibility and round-trip convertibility with other standards. They are variants of characters that already have encodings as normal (that is, non-compatibility) characters in the Unicode Standard; as such, they are more properly referred to as compatibility variants. Examples of compatibility variants in this sense include all of the glyph variants in the Compatibility and Specials Area: halfwidth or fullwidth characters from East Asian character encoding standards, Arabic contextual form glyphs from preexisting Arabic code pages, Arabic ligatures and ligatures from other scripts, and so on. Other examples include CJK compatibility ideographs, which are generally duplicates of a unified Han ideograph, and legacy alternate format characters such as U+206C INHIBIT ARABIC FORM SHAPING.

The fact that a character can be considered a compatibility variant does not mean that the character is deprecated in the standard. The use of many compatibility variants in general interchange is unproblematic. Some, however, such as Arabic contextual forms or vertical forms, can lead to problems when used in general interchange. In identifiers, compatibility variants should be avoided because of their visual similarity with regular characters. (See Unicode Technical Report #36, “Unicode Security Considerations.”)

The Compatibility and Specials Area contains a large number of compatibility characters, but the Unicode Standard also contains many compatibility characters that do not appear in that area. These include examples such as U+2163 “IV”ROMAN NUMERAL FOUR, U+2007 FIGURE SPACE, and U+00B2 “2” SUPERSCRIPT TWO. There is no formal listing of all compatibility characters in the Unicode Standard.

大雑把に説明すれば、

• 第1段落: Compatibility Characterとは、他の標準からとの互換性及び変換のために設けたものである (そうでもなければ入れていない)。Unicodeにはそれらの文字に対応する「普通の文字」(compatibility characterではない文字) が存在する (だから、どちらかといえばcompatibility variantと呼ぶほうが適切である、ということで以下そう呼ばれている)。例として、全角・半角文字のほか、互換用漢字やアラビア文字などを挙げている。
• 第2段落: ある文字がcompatibility variantであることは、この標準の中でdeprecated (標準化の文脈では「廃止予定」とか「使用を推奨しない」という意味) であることを示すものではない。多くの場合、compatibility variantを使うことに問題はない。ただし、一部のアラビア文字の場合に問題が出ることがある。
• 第3段落: Compatibility and Specials Area (U+F900～U+FFFF) の範囲には多くのcompatibility characterを含んでいるが、それ以外の領域にもある。ローマ数字のIVや上付き文字の2などがそれである。

…ということで。