The public API has several limiting factors. First is that there's no easy way to simply get a large list of users currently playing Halo. Microsoft doesn't make public player activity numbers public so having easy access to this would probably be against their intention. To get around this I looked at my recent game history, and took all the names of the players of my opponents. Then looked at all their recent game histories. Quickly I was able to get a list of a little over 17,000 unique names that played at least some form of matchmaking.
The second limiting factor is that Microsoft only allows 1 request per second. This isn't really a huge hurdle as I just left it running overnight to gather the data. It would be nicer if there was a faster option though.
Anyways here are some results from the Team Arena, Slayer, and SWAT playlists. Keep in mind that when this data was gathered it had only been about 10 days since the February season went live. I'll try to do something like this again at the end of the month to see if there are any major changes.
Team Arena:
From what I've heard of 343i's Halo ranking system a near normal curve like this is to be expected. With gold and platinum ranks containing the majority of players. There are some spikes at tier 1 of each rank. This is due to the fact that in Halo 5 you cannot rank down out of a division until the next season. So if players are improperly placed into diamond or go on a hot streak and make it into diamond, they'll just end up sitting at diamond 1 most of the season. With how quick the seasons in Halo are (historically slightly over a month for pre-season and January) I don't think this is that bad of an issue, as it does relieve a little bit of 'ladder anxiety' from the matchmaking knowing that you won't rank out of a division if you go on a losing streak.
Another interesting thing is the large percentage of players currently in onyx. I think this is likely due to my small-ish sample size of just over 7000 players for Team Arena, and because it's still early in the season. As the season goes on the more casual players will finish their placement matches and the relative amount of onyx players will probably decrease.
I've also included a boxplot showing win rates of players in the different ranks. Nothing really surprising; as players rank increases so does their win rate. Though from my understanding the matchmaking system should try and give players a close to 50% win rate, so again, perhaps this will even out as the season progresses.
Slayer:
Nothing too different in the slayer playlist other than it seems skewed towards higher ranks. I'm actually a little lost as to why this is. My best guess is that players who play Slayer might be more competitive than those that play Team Arena and combined with it being early in the season it's likely to produce these results. But again, I don't really know for sure. Maybe you have some suggestions?
SWAT:
Compared with the other two playlist there is hardly anybody in SWAT above platinum. From what I've heard 343i have done some tuning for ranks in SWAT which is probably why it looks so different than the others.
Well I hope you enjoyed these. Comments and suggestions on what other kinds of data from Halo 5 might be interesting are definitely welcome. What I really want to do is make some heatmaps (still) but 343i seems like they do not want to make the necessary data to do this available. Maybe soon... hopefully.