[Stealth should have an active and a passive mode. The active mode provides a higher chance of success but reduces speed by one half or so. Active modes ends when combat starts.]
I'd go for a key to turn it on and off.
No need o turn it off in combat - if you are trying to sneak if here are people/monsters aware of you, you'd better stop or it's bad for you.
Compared to ADOM, the look command should maybe provide more information about if a monster is aware of your presence, suspicious, calm, etc.
[Higher level monsters and certain animals should be immune or more resistant.]
I wouldn't stretch this as far as immunity. How about magical means to both conceal yourself (Non-detecton spell, camouflage spell, as opposed to invisibility) as well as revealing sneaking characters (True sight spell, Clairvoyance, Necklace of the eye)