open file: Scripts/Misc/CurrentExpansion.cs

edit this line:
public static readonly Expansion Expansion = Config.GetEnum<Expansion>("Expansion.CurrentExpansion", Expansion.TOL);

to whatever expansion you want to use
public static readonly Expansion Expansion = Config.GetEnum<Expansion>("Expansion.CurrentExpansion", Expansion.AOS);

this will remove all mobiles, items, art, etc from the game that are post-AoS. you may have to edit some spawns.
 
AoS is before elf/garg races. Setting the expansion to that should disable players being able to choose them
 
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