Years ago the issue of drainage grates was a more serious problem than it is right now. There has been great progress with the industry making grate openings small enough so they don’t allow a bicycle tire to go into them.
But some of these old grates are still out there and some of them are in places where a cyclist wouldn’t expect them. And, also it’s not simply the grate but it’s the box, the concrete or frame around it that is the problem. If that isn’t constructed properly, if the contractor leaves a space on either side large enough for a wheel to fall into it, a cyclist will be seriously injured.
While the grates have improved they are still a threat to cyclists.