Discord.js: messageReactionAdd event not triggering if message has been sent before bot start

[NLDev]

Please describe the problem you are having in as much detail as possible:

I am not sure whether this is by design, but the messageReactionAdd event only fires on messages which have been sent after the bot has started. So, if I add a reaction to an older message, the even won't get triggered.

See here: https://github.com/discordjs/discord.js/issues/1279#issuecomment-351171836

Include a reproducible code sample here, if possible:

"use strict";

let Discord = require("discord.js");
const client = new Discord.Client();

client.on("messageReactionAdd", (messageReaction, user) => {
    console.log(messageReaction);
});

client.login("foo");

Further details:

• discord.js version: v11.3.2
• node.js version: v8.11.3
• Operating system: Ubuntu 16.04

• Priority this issue should have – please be realistic and elaborate if possible:
  Not quite sure. This isn't a major bug I guess.

• [ ] I found this issue while running code on a __user account__

• [ ] I have also tested the issue on latest master, commit hash:

[RighteousPotato]

This is by design. See here for how to get around it:
https://discordjs.guide/#/popular-topics/reactions?id=listening-for-reactions-on-old-messages

[NLDev]

Alright, thank you very much! :)