The Matrix specification supports spoilers since version 1.1:
Automaattinen sisällysluettelo / Automatically generated Table of Contents
Best case scenario
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.
Manual
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>
IRC
Spoilers are also supported by IRC, but not matrix-appservice-irc.
Related issues
Privacy Badger will take care of third-party tracking the best, but hiding this message takes a more targeted filter such as uBlock Origin (for Firefox) or uBlock Origin Lite (for everything else).
Additionally considering adblocking DNS services such as DNS4EU Protective Resolution With Ad blocking or public AdGuard DNS may be a good idea to clean up your modern internet.
Learn more about targeted advertising!
PS. I am not sorry if this cosmetic filtering targeting script doesn't detect your protection, because this message brings me too many amused messages and smiles for that (and thus this message is here to stay) 🐾