I'm trying to use formatting in the messages. However this doesn't seem to work anymore.
It worked fine in 1.18.4
Broke since 1.19.0 (and dev build)

Example:

MinecraftPlayerJoinMessage: 
  Enabled: true
  Webhook:
    Enable: false
    AvatarUrl: "%botavatarurl%"
    Name: "%botname%"
  Content: ""
  Embed:
    Enabled: true
    Color: "#00ff00"
    Author:
      ImageUrl: "%embedavatarurl%"
      Name: ":tada: **%username% joined the server**"
      Url: ""
    ThumbnailUrl: ""
    Title:
      Text: ""
      Url: ""
    Description: ""
    Fields: []
    ImageUrl: ""
    Footer:
      Text: ""
      IconUrl: ""
    Timestamp: false

Using formatting in the DiscordChatChannelServer(Startup/Shutdown)Message however works fine.

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You have Enabled: false

Yes because I disabled this one. I used this one as a test for the MinecraftPlayerFirstJoinMessage message. Because I don't have that many new players to test with

Another example which doesn't work

MinecraftPlayerLeaveMessage: 
  Enabled: true
  Webhook:
    Enable: false
    AvatarUrl: "%botavatarurl%"
    Name: "%botname%"
  Content: ""
  Embed:
    Enabled: true
    Color: "#ff0000"
    Author:
      ImageUrl: "%embedavatarurl%"
      Name: ":tada: **%displayname% left**"
      Url: ""
    ThumbnailUrl: ""
    Title:
      Text: ""
      Url: ""
    Description: ""
    Fields: []
    ImageUrl: ""
    Footer:
      Text: ""
      IconUrl: ""
    Timestamp: false

the reason it "doesn't work" is probably because you have the silent join & leave permissions (discordsrv.silentjoin / discordsrv.silentquit), join the Discord server if you need further assistance

You miss the point.
The message works fine except the formatting refuses to work.
I found out I'm not the only one https://www.spigotmc.org/threads/discordsrv.123540/page-72#post-3829829
But nvm. It's useless to argue.

That post you linked was someone asking to revert to the old non-embed system, which was answered by me just below that. You haven't yet shown a single piece of evidence showing something not working or a error, as far as I can see there's absolutely nothing not working here.

There is not a single error

the author name in embeds doesn't support formatting on Discord