When I launch this plugin in my 1.18.1 server, I receive the following error in the console:

[22:26:19] [Server thread/INFO]: [Magic] Enabling Magic v10.4-58a8529
[22:26:19] [Server thread/INFO]: [Magic] EffectLib initialized
[22:26:20] [Server thread/ERROR]: [STDERR] [org.bukkit.craftbukkit.v1_18_R1.legacy.CraftLegacy] Initializing Legacy Material Support. Unless you have legacy plugins and/or data this is a bug!

Will this error effect the plugin or should I ignore it?

It can not be removed without dropping backwards compatibility. probably at some point in the future. I don't think this has a performance impact.

I too get this error, from this plugin:

[15:18:52 ERROR]: [STDERR] [org.bukkit.craftbukkit.v1_18_R1.legacy.CraftLegacy] Initializing Legacy Material Support. Unless you have legacy plugins and/or data this is a bug!

Can this legacy material usage be removed? It probably has a performance impact.