FreeGameDev

This would allow hidden text using [spoiler] tags.

https://www.phpbb.com/customise/db/mod/ ... e_spoiler/

Is there a need?

Probably no need for a mod by the way.

Yes, there's a need.
[spoiler]Neko has breast implants[/spoiler]

There's other reasons, like including a stack trace or a big image etc.

Or including a large volume of text, possibly story related. Blocking out in grey is a hack.

wrong tool

links to examples could help come up with solutions.