/effect slowness doesn't work on player in spectator mode.

I expect this is intended behaviour.

On the annouce of the spectator mode mojang said : "The mouse scroll wheel, the sprint key, and the slowness and swiftness effects all affect the flying speed.", so no this is not intended.

(Sorry for my english, I'm french.)

P.S. : Sorry for duplicate but I didn't find my post even in searching "spectator mode"

That's not what Mojang said. That's what the wiki said, and its not meant to be used as a source for bug reports.

So if I understand you say that is normal that swiftness work and slowness not ?
Why all effects except slowness work on spectators ?
If the wiki isn't a source to know how it's supposed to work what is the source ?
I understand that the wiki can be modified etc, but when I tried to immobilize players for my system and I see that it's not work I searched and I didn't see anywhere that slowness doesn't affect spectators...
What I supposed to do ? If no one talk about it that's will never be change if it's supposed to be changed...
Where can I see if it's actually a bug or not ?

The wiki is a good community-run source of information, but it may sometimes document information that is not correct. That being said, there is no confirmation at present as to whether or not this issue is a bug. Until such confirmation is made, this report is remaining open.

Does /effect 1 10 10 work?