graph node insert -foo ;# works
g walk -foo -command {apply {args {puts stderr [info level 0]}}} ;# results in

apply {args {puts stderr [info level 0]}} enter ::g -foo
bad restriction "-foo": must be -adj, -embedding, -filter, -in, -inner, -key, -out, or -value
while evaluating {g walk -foo -command {apply {args {puts stderr [info level 0]}}}}

Note how apply is invoked, i.e. [walk] is not directly affected, but one of the internals that it calls.

I know that fixing this will be quite some work, but it is an actual restriction: e.g. the package does not work for negative numbers as node names