ונוהגין להפטיר בחתונה שוש אשיש.
The Rama above (from OC 428) says that in his area the custom was to read a special haftora if a groom was present in the synagogue.
Has anyone seen this minhag or know why it seems to have largely died out?
Wikipedia claims (with no source given) that
No one reads a special haftarah for a bridegroom any longer, except the Karaites.
This is a revised edition of something I myself added to that article in 2004. While I didn't cite any source (and, still, there's no citation for that claim in the Wikipedia article), I do remembering having researched the issue, and I feel confident that my self of several years ago would not have added that information without its being IMO reliable; alas, I no longer remember what the source was.
Edit: Note, though, that Shaare Efrayim 9:26 says that "some places" do read this haftara but that "we" do not.
In "Minhagim of Worms", it says as follows:
"Nissuin on Shabbos - Brautloft
When a Chupah takes place on Friday, Shabbos is called Brautloft (Bruitloft, Breiluft). [This was not a common occurrence, as discussed earlier, but was done at various times]. (FN 21 - The word Brautloft is translated as wedding, although the actual Chupah ceremony was on Friday, not Shabbos. This Shabbos is also known as שושבינות (Shushbinus). It is important to stress that all references in the Rishonim to Shabbos Sheva Brachos do not refer to the Shabbos known as Schenk Wein, which refers to all Shabbos Sheva Brachos where the wedding occurs on any weekday except Friday. The Brautloft that the earlier sources refer to is when the Chupah takes place on Friday and the actual meal and rejoicing take place on Shabbos. All Yotzros for weddings that are printed in older prayer books, the special Haftorah of Sos Asis that is discussed in the sources, are specifically for this specific Shabbos known as Brautloft. Many people have erroneously confused these two Shabbosos)."
In "The Spirit of the Maggid", Rabbi Krohn quotes Rabbi David Cohen that it's "because communities had become so large, that there could be Chassanim in Shul every week and the Sos Asis haftarah would have to be read so many times while the other Haftaros would be left out".