there is really no reason why this shouldnt exist

Telegram recently implemented these and Discord had it for some time already, it's kinda useful for a lot of stuff.

With this starting back in 2016, do these votes still even mean anything anymore?

Add a spoil encapsulation feature to the chat to prevent spoil.

This would be cool. Could have it with a /s command before a message, and reveal spoilered messages on mouse-hover-over-chat.

Twitch in some form already supports Markdown within Twitch profiles. I am not sure I see why they couldn't bring this functionality to chat for those that understand markdown. It would also encourage more users to use and understand markdown which in turn could increase its use across other platforms such as Discord.

This would be great. I was made a mod to delete spoilers for a game but then the streamer just saw the spoilers more because they were highlighted as deleted and he checked them.

Edit: Sinspitterx said it more clearly than me. Mods need this ability and chatters would be able to enjoy chat more.

This would actually be a really good feature. I've wanted to make references to spoilers in a game to people in chat that have already played it, but couldn't because it would ruin it for others.

Eyyo why has no one done this yet? It's 4 years. Spoiler function like what discord has would allow viewers to talk about the game in a no backseat/blind playthrough. Greatly helps viewer experience. And make sure it's streamer toggleable so people aren't doing dumb things like jebaiting to not annoy the streamer.

Not really sure when this would be useful.

The idea behind flagging messages as spoilers is allowing people to have a discussion without ruining the experience for others.

Mods would be able to flag a message as a spoiler, making the message hidden for the user until they click on it to reveal it.

Another idea would be a command for mods to set the channel to "spoiler sensitive" which allows viewers to self-flag their messages as potential spoilers if they are not fully sure about what is allowed to talk about or not.
Having this only allowed only whenever the channel is specifically set to this mode (for story heavy games or newly released games) would keep the abuse this function down. Also mods would be able to see the messages and could see people potentially abusing the function and then can proceed with other already given ways to deal with said person.

Example usage:

Mods usage could be
- /spoileron => "The room has been set to spoiler sensitive"
- /spoil username 2 (for the last two messages to be flagged)

Users could be
/s Has streamer X already reached part Y and beat Z?

Sounds like it would be abused by trolls and not very useful as chat is always moving up.

Spoiler Tagging would be one of the biggest upgrades to the overall Twitch experience for a lot of streamers, especially folks doing Let's Plays. Right now the only way to deal with viewers posting spoilers is to time them out or clear chat, which isn't a positive viewer experience. Letting viewers opt into mouseover spoiler tags and letting mods apply spoiler tags to existing messages from other viewers would be an invaluable tool, and it would let chats for games sensitive to spoilers be much more active. Right now, there is a "walking-on-eggshells" fear for a lot of games and channels that doesn't promote a healthy chat environment.

Example implementation would mirror the current whisper functionality:

Viewers type:

/s "Spoiler Context" Message Text

To post their message to chat, blacked out, with visible overlaid text suggesting viewers must mouseover/click/tap to view the spoiler.

If viewers specify a "Spoiler Context" argument, this would also appear in the visible overlaid text. If they don't specify that argument (e.g. just type /s Message) a default context that just says "Spoilers" will display.

Mods should be able to mark existing messages as spoilers by clicking them and by using a separate chat command to mark recent messages from a viewer as spoilers.

I would love to see a way to conceal spoilers in the chat. That way mods don't have to struggle as hard to police them in blind streams, and chatters aren't as limited on what they talk about when in a stream with a blind streamer.