https://bugs.winehq.org/show_bug.cgi?id=48507
--- Comment #7 from Nguyễn Chính Hữu huupoke12@gmail.com --- By tracing gdiplus with WINEDEBUG, it seems that osu! is using Font Awesome (I think it's Font Awesome 4) to display the icons. The version 4 make use of Unicode private use area to store the glyphs (https://fontawesome.com/v4.7.0/cheatsheet/).
So I have tried downloading Font Awesome (4.7.0), changed its name to Tahoma and placed it in Linux's font directory (in order to make the in-game chat use Font Awesome). Then I have copied the characters from the above page and pasted them into the in-game chat, they does not show up (just invisible, no tofu, no character space) but they still take up "backspace" keys to delete (which means if I pasted an icon after a normal character and press "backspace" to delete, it delete the icon first (even invisible), then the normal character. But when I tested in Wine Notepad and Notepad++, they shown up normally.
So I guess it's maybe gdiplus doesn't handle that Unicode range properly, or maybe something related to reading the font metrics (such as displaying glyphs at extremely small size so they're just invisible), or something else.
By the way, in the #6 comment, it's not "fixed". It happened because the game now doesn't use its custom font (Aller) to display the UI anymore, instead it use Tahoma, and Tahoma have those characters so those characters are displayed. Reverting the file `gdiplus/font.c` to its Wine 5.0 version make it behaves like before, so I would call that a regression.