This is assuming that the tf2 community as a whole is unbanning weapons that are not annoying or overpowered for some end goal with valve, which has been talked about a lot in other discussions.
Before scrimming with the solemn vow I thought the same thing, seeing all of the enemies' health is super overpowered and removes a key aspect of the game: keeping track of ubers and health.
But after scrimming with the solemn vow for a few days now, it does not nearly have the same impact as you are describing. First off, you said that teams can be 100% sure of whether they can take a fight or not and the meta game of teams trying to keep track of enemy health will be discarded almost completely. There are very few times, not including stalemates where it can be assumed that enemies are fully buffed, where the medic calling out an enemies health will be beneficial. If there is fighting going on, the medic's main job, usually, would be healing his teammates for as much as possible, arrowing whenever possible. To do this the medic must be looking at his teammates very frequently through switching heal targets, arrowing, and buffing the people behind, so the medic's only opportunity to see enemies' health is when he is in the middle of healing a teammate moving forward. The medic must also be in a position to see an enemies health, which granted they would be fighting, isn't usually too difficult. So let's say the medic does get an opportunity to see the enemies' health. He would be able to see maybe 3 enemies' health at max. In a fast moving fight, the health is constantly changing and by the time you are able to call out someone is lit, a teammate should have called their damage - or at least some sort of damage call - and a teammate would already be prioritizing their shots on that specific enemy. So, in fast moving fights the solemn vow is very rarely viable except when determining enemy medic's ubercharge, which in higher level play, you should already have an idea of, and in lower level play the medic should be more focused on keeping teammates alive as it is usually very overwhelming to keep track of your teammates health. But lets say that in higher level play you do happen to lose track of ubers and the enemy team has ad. To get a sight line on the enemy med, your team must first clear traps, and then you would be able to peek out to spot their med. This would rarely be able to happen because the enemy team with ad will see you guys moving up and while your team is playing close, they will uber on you and completely fuck your team even if you do somehow spot there med. In very few spots in very few maps would a team with ad not be playing close enough to uber on you if you do get the call, but in those cases, yes, the solemn vow would be useful. So the solemn vow is not effective during most fights and trying to call out uber advantage.
However, the solemn vow is effective when fights are starting to close and there are fewer people you have to keep track of, so you are able to see enemies' health which your team probably does not know as it is the end of the fight and a lot of damage/heals has been traded. Also, seeing if the enemy is running kritz (disregarding quick-fix as that weapon is likely gonna get banned) is super powerful, but very rarely happens as they will likely have used kritz (if they have it, which they should because the enemy should only really run kritz if they would have ad) by the time you are able to see the med because if they were pushing, they would have used around the choke, and if you were pushing would have used while your combing was pushing through the choke. The solemn vow is also effective when getting chased so you could determine how to fight the enemy, but I would say the ubersaw is just as effective, although possibly more effective but higher risk.
Overall, since the med would very rarely be able to effectively help his team call damage during most fights, it is not nearly as overpowered as you are describing it, and, although slightly better than the ubersaw, should remain unbanned as it is not really annoying or overpowered. (It does however, slightly lower the skill cap of med as he no longer has to keep near as much track of enemies health as before)