The OGS developer IS using ELO rating given in that article: ELO rating VS rank on OGS - #3 by matburt
They’re just applied in different categories separately.
I think most of us believe that the current system has some problem in the ranking system (otherwise there would not be so many complaints). What we are not sure is how we should fix these problems.
Solution 1: Use only one rating for all categories, at the cost of some inaccuracy or ignoring the rankings on the variations.
Most other websites do this, but as temifar pointed out, most sites have a strong bias towards either live or correspondence games. From my personal experience:
(1) LittleGolem counts all board sizes equally with the same weight, and as a result, the 9*9 bot Valkyria9 is over 10 dan: Little Golem
–Edit Begin–
//(2) DGS only counts traditional 19*19 games as rated games, and all other variations are unrated.
Thanks to @temifar for pointing it out that the statement above is not accurate. Instead,
(2) DGS tournaments tend to be unrated for non-standard games, but normal single games can be rated for any supported board size: DGS - FAQ
–Edit End–
Personally, I think the factor of the board size is more important than the factor of time here.
Solution 2: Keep splitting the categories, but improve the way it works.
Specifically, I think the provisional status should be enabled in each specific category if
(1) the player has not played enough games in that category, or
(2) the player has not played in that category for a long time and the ranks of the player in the other categories are different from the rank in this category a lot.
As SanDiego pointed out, a warning should be added in such cases.