It doesn’t change much with time.
For season 3, the rank distribution was Bronze: 6%, Silver: 22%, Gold: 34%, Platinum: 23%, Diamond: 10%, Master: 3%, Grandmaster: <1% (Overwatch Forums). For season 8, the rank distribution was Bronze: 8%, Silver: 21%, Gold: 32%, Platinum: 25%, Diamond: 10%, Master: 3%, Grandmaster: 1% (Competitive Mode Tier Distribution).
The ranks are probably similar today.