It is mentioned here:

https://unicode.org/faq/casemap_charprop.html#6a

> The Unicode Standard encodes these two compatibility characters [0x0132 IJ and 0x0133 ij] to provide support for roundtrip conversion of the Dutch letter 'ij' in certain very rare legacy (non-Unicode) character encodings. It is strongly preferred (and far more common) to use the two character ASCII sequence 'ij' to represent this letter instead.

You can dig in the Unicode mailing lists for discussions on this from over twenty years ago. The bottom line is that you shouldn't use 0x0132 and 0x0133 in modern text. By now this is a resolved issue.

I'm not sure this issue can ever satisfactorily be resolved by ignoring it. Yes, it is customary even among Dutchies to just type i-j because it's easier, but the Dutch alphabet does have a dotted ij as its 25th letter. It's not a ligature in the source language, it is a single letter; as evidenced by the fact that the proper noun IJsselmeer (among others) is written with two capital letters (if not using the single 0x0132 IJ). The letter y does not exist in the Dutch script, only in loanwords (of which we have plenty).

Stop repeating that nonsense! The current Dutch alphabet has 'Y' as its 25th letter. Not 'IJ'. This is a historical thing.

The 'IJ' is a letter, culturally speaking, in the sense that it is capitalized as one, and that it is often rendered as a single unit (which you can see in vertical lettering, if done right), and sorted as if it was a single letter. In terms of character encoding however, it is a 'i' followed by a 'j'.

Cf.:

* https://onzetaal.nl/taalloket/ij-plaats-in-alfabet (first sentence!)

* https://taaladvies.net/ij-alfabetisering/

See my other posts for 0x0132, which is a compatibility character for rare obsolete character encodings which did encode it on a single codepoint.

I corrected the Wikipedia article now. That code point is not deprecated, it's a compatibility character which is a different thing entirely.