Why?

I've seen this on a different server, which makes it clearly visible so (long time) players can see that someone has linked their account.

What and How?

A "◆" can be added as a suffix to a player's name.
Simple have a placeholder for it.
If player is linked, they get that symbol (that can be customised), else leave blank.

So I can add it to the prefix section of the permission group, and it'll work as intended.

Are there alternatives?

To not add it I guess xD

Checks

• I have used the search at least once to check if my idea has already been suggested and perhaps already implemented.

Anything else

Would be pretty cool.
Also I can send screenshot of it or give ip or the server.

This can currently be accomplished by use of the MinecraftDiscordAccountLinkedConsoleCommands option. Try something like:
MinecraftDiscordAccountLinkedConsoleCommands: ["nick %minecraftplayername% %minecraftplayername% <symbol>", "", ""]

If you're using LuckPerms you can use the discordsrv:linked context, eg.

/lp group default meta addsuffix 100 "◆" discordsrv:linked=true

or alternatively you could use the %discordsrv_user_islinked% placeholder with PlaceholderAPI's javascript expansion

If you're using LuckPerms you can use the discordsrv:linked context, eg.

/lp group default meta addsuffix 100 "◆" discordsrv:linked=true

or alternatively you could use the %discordsrv_user_islinked% placeholder with PlaceholderAPI's javascript expansion

So will the older users recive it?

So will the older users recive it?

Yes, those two methods don't rely on being applied before the users linked