While I do agree that one of the harder things for new players to grasp is what is safe and what is not, I think that having a visual indicator would make the game much busier than it should be. The reason I say this is that most exchanges would show something like “Punish!” (or “Counter” or “Crossup” or “Jumpable”) At this point, players would probably start tuning out all of the visual indicators instead of trying to help cue in on some of them. For instance, counter and crossup are very useful feedback for immediate decisions. When you get the counter popping up, then you know you can follow with the counter combo. If you had to distinguish between a punish versus a counter, that would slow down your decision. The “Jumpable” is also useful because, in general, you do not have to immediately follow-up on a situation that shows “Jumpable”, and it is not obvious at all that you need to jump to get out of command throws.
The other reason I am not sure if I would want a “Punish” indicator is that I don’t think it would make the situation more obvious as to what the player needs to do. If you do a move, and then you get hit afterward when someone blocks it, then you already have an immediate feedback. It’s just that many new players aren’t in the mindset of actually having a defense. So, they don’t even utilize that feedback they are getting. Most of the problems that I see with new players (and I play with new players all the time) is that they don’t get the concept of defense. It’s not about the concept of safety. Until they understand defense, they don’t begin to comprehend the concept of “safe” attacks. When I point out the basic concept of how exchanges generally flow in a fighting game, most of the new players that I play usually figure out what attacks are safe and which are not because they get into the mindset of “Oh, I should probably block now since my attack was blocked.” If their block fails, they pretty quickly figure out that they couldn’t block after that move.
Edit: I just realized I missed an important point you stated in the first paragraph. Untechable throws is a good situation to have a “Punish!” indicator. It would make it more obvious to the attacker that they aren’t able to Yomi the throw since they are still in recovery, and the situation would not require an immediate follow-up since you are going through a throw animation anyway.