NFHS softball unreported substitute(s)
 

Mid-game, coach decides to replace all 3 outfielders but does not report the substitutes to the umpire.

After 1 out, the opposing coach brings the unreported subs to the umpire's attention. After confirming with the offending coach, the 3 subs are properly added to the lineup.

Does this constitute only a verbal warning or a bench restriction?

It probably should be a single warning, treating the violation as a single occurrence, because the coach made one big mistake.

It matters that the opposing coach reported it.
NFHS 3-6-7 PENALTY