diff --git a/src/systems/abstractsystem.jl b/src/systems/abstractsystem.jl
index 248768b8d4..d301f2ae23 100644
--- a/src/systems/abstractsystem.jl
+++ b/src/systems/abstractsystem.jl
@@ -1019,12 +1019,7 @@ function Base.getproperty(
 end
 function getvar(sys::AbstractSystem, name::Symbol; namespace = does_namespacing(sys))
     systems = get_systems(sys)
-    if isdefined(sys, name)
-        Base.depwarn(
-            "`sys.name` like `sys.$name` is deprecated. Use getters like `get_$name` instead.",
-            "sys.$name")
-        return getfield(sys, name)
-    elseif !isempty(systems)
+    if !isempty(systems)
         i = findfirst(x -> nameof(x) == name, systems)
         if i !== nothing
             return namespace ? renamespace(sys, systems[i]) : systems[i]
@@ -1079,19 +1074,14 @@ function getvar(sys::AbstractSystem, name::Symbol; namespace = does_namespacing(
 end
 
 function Base.setproperty!(sys::AbstractSystem, prop::Symbol, val)
-    # We use this weird syntax because `parameters` and `unknowns` calls are
-    # potentially expensive.
-    if (params = parameters(sys);
-    idx = findfirst(s -> getname(s) == prop, params);
-    idx !== nothing)
-        get_defaults(sys)[params[idx]] = value(val)
-    elseif (sts = unknowns(sys);
-    idx = findfirst(s -> getname(s) == prop, sts);
-    idx !== nothing)
-        get_defaults(sys)[sts[idx]] = value(val)
-    else
-        setfield!(sys, prop, val)
-    end
+    error("""
+    `setproperty!` on systems is invalid. Systems are immutable data structures, and \
+    modifications to fields should be made by constructing a new system. This can be done \
+    easily using packages such as Setfield.jl.
+
+    If you are looking for the old behavior of updating the default of a variable via \
+    `setproperty!`, this should now be done by mutating `ModelingToolkit.get_defaults(sys)`.
+    """)
 end
 
 apply_to_variables(f::F, ex) where {F} = _apply_to_variables(f, ex)
diff --git a/test/model_parsing.jl b/test/model_parsing.jl
index 88fa7faac7..a615a1d616 100644
--- a/test/model_parsing.jl
+++ b/test/model_parsing.jl
@@ -977,7 +977,7 @@ end
 
 @testset "Multiple extend statements" begin
     @named multiple_extend = MultipleExtend()
-    @test collect(nameof.(multiple_extend.systems)) == [:inmodel_b, :inmodel]
+    @test collect(nameof.(get_systems(multiple_extend))) == [:inmodel_b, :inmodel]
     @test MultipleExtend.structure[:extend][1] == [:inmodel, :b, :inmodel_b]
     @test tosymbol.(parameters(multiple_extend)) == [:b, :inmodel_b₊p, :inmodel₊p]
 end
diff --git a/test/symbolic_parameters.jl b/test/symbolic_parameters.jl
index 6a6a434ccc..15b19b320a 100644
--- a/test/symbolic_parameters.jl
+++ b/test/symbolic_parameters.jl
@@ -21,7 +21,7 @@ u0 = [
     z => u - 0.1
 ]
 ns = System(eqs, [x, y, z], [σ, ρ, β], name = :ns, defaults = [par; u0])
-ns.y = u * 1.1
+ModelingToolkit.get_defaults(ns)[y] = u * 1.1
 resolved = ModelingToolkit.varmap_to_vars(Dict(), parameters(ns),
     defaults = ModelingToolkit.defaults(ns))
 @test resolved == [1, 0.1 + 1, (0.1 + 1) * 1.1]
@@ -33,8 +33,8 @@ sol = solve(prob, NewtonRaphson())
 @variables a
 @parameters b
 top = System([0 ~ -a + ns.x + b], [a], [b], systems = [ns], name = :top)
-top.b = ns.σ * 0.5
-top.ns.x = u * 0.5
+ModelingToolkit.get_defaults(top)[b] = ns.σ * 0.5
+ModelingToolkit.get_defaults(top)[ns.x] = unknowns(ns, u) * 0.5
 
 res = ModelingToolkit.varmap_to_vars(Dict(), parameters(top),
     defaults = ModelingToolkit.defaults(top))