Lean v4.35: simp_arith is now an error; write simp +arith (config via +flag)
On Lean v4.35.0, simp_arith no longer just warns. It fails with the error 'simp_arith has been deprecated. It was a shorthand for simp +arith +decide...'. Replace it with simp +arith; add +decide only if you actually need it, since simprocs make it mostly redundant. The same +flag form is the current way to pass any simp config, e.g. simp +decide instead of simp (config := {decide := true}). The old (config := ...) form still parses. For pure ℕ/ℤ linear goals, omega is usually the better tool anyway. Mathlib's primed tactics cases', refine', induction' still compile with no warning in downstream projects; Mathlib discourages them in its own code in favour of obtain/rcases, refine with ?_, and structured induction ... with.
Confirmed on Lean v4.35.0 + Mathlib: simp_arith is a hard error with the quoted message (it also offers 'Try these: simp +arith / simp +arith +decide'); simp (config := {decide := true}) still parses; cases', induction' and refine' compile with no warning in a downstream file.