It's not just deprecation, it's systemic understatement. It drives non-British people insane because everyone is talking in code.

And some of the meaning is hidden in intonation.

If someone says "Interesting..." that can mean "That's the dumbest thing I've ever heard" or "Might be worth a look, but not a priority right now." Or maybe "That's very suspicious."

"That's quite good" usually means "Very good, I like it!"

There is the famous case in the Korean War at the Battle of Imjin River where the British commander of the Gloucestershire regiment reported to an American General, 'Things are a bit sticky, sir'. The American General thought that meant a good thing, like they were holding the line, when in fact they were fighting a heroic last stand outnumbered 25:1!

https://www.warhistoryonline.com/korean-war/battle-of-imjin-...

I certainly use the word "exciting" in ways that that might be non-standard, like for instance describing when everything has gone catastrophically wrong.

Also starting anything with "With the greatest respect..."

> If someone says "Interesting..." that can mean ...

Same in German.

> "That's quite good"

Ummmm...... I'll say it is "not bad".

For me to say something is "quite good" it would have to make me cream myself.

'Interesting' => 'you're stark raving mad but you're in the room with me so I'm going to be polite to you until I'm out of striking range'.