It's interesting to me that this happened far enough back that the only time we tend to consider 'th' to be letterlike is when we need to explain pronunciation — e.g. when reaching a kid to read
Of course then you'd have to teach people to use that character when writing instead of just typing "ij". And one day, there will be someone who, for stylistic reasons, needs tight control over when it's rendered as "ij" and when as "ÿ"...
Her is a link with some nice information: http://www.uazone.org/multiling/euroml/annex02.html
Based on precedent, I'd expect the Unicode Consortium to declare it the radical of a Chinese character.
(it does exist in Unicode, but as a ligature -- U+0132 for majuscule, U+0133 for minuscule -- and decomposes to the two code points for 'i' and 'j', with use of the ligature discouraged)