mikaela.github.io/n/matrixspoilers.md

The Matrix specification supports spoilers since version 1.1:

• Spoiler messages at Client-Server API of Matrix Specification

My favourite Matrix clients, FluffyChat and Nheko, support spoilers when the spoiler is in the following format; ||Reason why this is a spoiler|Actually spoilered text.|| It can also be in-line and a spoiler without reason is just ||spoiler goes here||.

Element Web supports whole message spoilers through the /spoiler command.

Other clients supporting formatting may have to enter the HTML directly e.g.:

<span data-mx-spoiler="REASON HERE">Spoilered text</span>

Some clients, like Nheko, have a /plain command to disable formatting for that message which allows sending this message in plain text to help someone else.

/plain <span data-mx-spoiler="REASON HERE">Spoilered text</span>

Spoilers are also supported by IRC, but not matrix-appservice-irc.

• matrix-org/matrix-appservice-irc#1548: Feature request: bridge spoilers to IRC by sending them with the same background&foreground colour (and that to Matrix as spoiler)