After the recent software change the next update is neccessary.
The new software doesn't support most of the old formatting codes anymore. Instead it interprets the well documented bbcode (Bulletin Board Code, introduced 1998, see Wikipedia). Unfortunately it uses only a (very) small subset. Only six tag pairs are recognized, of which two have (at this moment) no effect.

​This are the fully working tags:
[​b​]​{text}​[​/b​]​
  {text}
  Makes {text} bold
​[​i​]​{text}​[​/i​]​
  {text}
  Makes {text} italic
​[​u​]​{text}​[​/u​]​
  {text}
  Underlines {text}
​[​code​]​{text}​[​/code​]​
  

{text}

  Meant for rendering code snippets

This are the working tags without function:
​[​s​]​{text}​[​/s​]​
  {text}
  Should create a strikethrough on {text}
​[​spoiler​]​{text}​[​/spoiler​]​
  {text}
  Should Creates a spoiler section (a box) that is by default collapsed until you click a button or the box itself to expand it to show the {text}

From the old tags has only the board internal linking survived:
>​> (two closing angle brackets) with the number of an existing post in the same board gives a link to that post, like >>5708.
​The links are striked through if the destination does not exist.

The two other closing angle bracket tags seem to be switched off:
> (one closing angle bracket) at the beginning of a line gave a quote.
>​>​> (three closing angle brackets) were used to link to an existing post in a different board, >>>/3dcg/45317.
As you can see, there is no more formatting or link action. At this moment I again know only the the long form, e.g. https://boards.guro.cx/3dcg/res/43846.html or https://boards.guro.cx/3dcg/res/44797.html#44957, to link to other boards.


Anticonfusion hint: The characters of the describing samples are preceded by, interrupted by or enclosed in "Zero Width Spaces" (​ or ​) to prevent the conversion.
Because of their nature you can't see these characters, but you can detect them indirect: It takes more than one stroke of the cursor left or right keys to move the cursor from one viewable character to the next.
You can test this here: 123456 (no Zero Width Spaces, six keystrokes), 12​45​​89 (one Zero Width Space between 2 and 4, two Zero Width Spaces between 5 and 7, nine keystrokes)
To copy Zero Width Spaces it is helpful to include the leading and following characters and delete these after inserting.
​

Special characters like arrows (←, →), horizontal ellipses (…), dashes (–, —) or even Zero Width Spaces (​) can still be inserted by copy and paste. Just get them from an old post or — like other characters — from the character table programm on your computer or phone.
Hint: It is always a good idea to check if the operation worked as intended. Sometimes characters are altered while moved to or from the clipboard. And sometimes different characters just look like they are the same, because they are presented similar by a program.


You are still invited to provide information about now working, reenabled or (for the archive) "historical" formatting codes to be added.