Java Edition:Long Sign
Signs from older versions could have more characters than currently possible.
Obtaining
15 Character Limit
From Infdev 20100607, when signs were fully implemented, until 14w21b, signs used a simple 15 character limit per line. In 14w25a, a new system was put into place that limited the number of characters per sign based on their width. By using 15 very long symbols in the same line before the new sign limit system, you can get a sign that would be impossible with the character size limits. Unfortunately, this sign will no longer show symbols that would hang off the margins of the sign, but they are still there. This means it is a unobservable discontinued feature, as the only way to test that it has these symbols is by using commands or external software. The longest characters that can be put on a sign include "~", "_", and "m."
Quotations
In 15w31a, every time signs are reloaded, they get quotations due to a bug.[1]
Languages
By choosing the option "Force Unicode Font" in the language settings menu to ON, more characters can be typed into a sign than with the standard font due to this option reducing the visual size of characters.
Resource Packs
In 1.13-pre6 and above, a resource pack can be used to set the width of characters smaller than usual. This allows for up to 65,522 characters per line before leading to errors during saving to NBT in singleplayer, or 384 characters per line in multiplayer due to packet size limitations. Going beyond this limit can lead to Chunk Regeneration#Large Sign (Singleplayer Only).
One can find a premade resource pack to change the font size to 0.0001 here.
Gallery
A sign with 15 Ms on each line
The block data of a long sign with tildes viewed through commands to prove it is discontinued
