I do not know why but for me the ping bar says the server is outdated when I enable and edit the custom slot.

"The connection bars turn red in the serverlist with VersionText on.": Have you ever seen when your server is outdated in the serverlist and displays some text near the signal bars? SMOTD simulates this and changes the text. This is just a cosmetical change and doesn't influence the functionality of your server.

https://github.com/strumswell/ServerlistMOTD-X/wiki/1.-FAQ#in-the-serverlist

Expected behaviour. The version text is just a hacky way of emulating an outdated server to the user and setting a custom text (of your config) instead. There is no other way afaik.

So, no way to fix that? Or not for now and you’ll release an update?

Because I think with the custom slot, the ping bar showing red looks kinda weird, considering that my server is running...

There is nothing to fix because this is how the version text (server protocol version) of Minecraft works. This is just the way it is. No update is going to change this behaviour. This is like complaining that hearts are red and not green.

Question answered.

Wait @strumswell, what about this...

What about the fact that the ping bars are there when the player count is modified?

This is a different state compared to the version text which I can not exploit to display a custom text. The "???" is triggered by the server but the text is hard coded on the client side.