[jamespullar]

Looks like you can still use https://discordmojis.com/ (at least for now) which was always mostly a mirror of Slackmoji.

[filmgirlcw]

Slackmojis.com still works. This was Slackmoji.com, which is different. It was an app that made it easier to upload custom emoji to Slack, rather than using their interface.