My favourite Matrix clients FluffyChat and Nheko support spoilers when
the spoiler is formatted like `||Reason why this is a spoiler|Actually
spoilered text.||` It can be in-line and reasonless spoilers are just
`||spoiler goes here||`.
Element Web supports whole message spoilers through the `/spoiler` command.
I am not sure on other Elements as I barely use them for other things than
`/devtools` command.
Other clients supporting formatting such as Nheko older than [60b1a595e121e6d824cab188abac750be4459058](https://github.com/Nheko-Reborn/nheko/commit/60b1a595e121e6d824cab188abac750be4459058)
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)](https://github.com/matrix-org/matrix-appservice-irc/issues/1548)