Humans go virtually extinct in short order. A century or two at best. The last human dies a couple of centuries after that.
As soon as the labor shortage becomes significant (under 50 years), health care systems break down, and soon after there is mass starvation. World population goes down far faster than natural rate. Once the population gets down to levels supportable by nature, the rate of decline goes down by a bit, but you are still taking a digit off the population every 25 years. And most people aren’t even living until they’re 50. So you’ll quickly get down to perhaps 100M, and then it’s just 8 generations after that that the last person dies.
There’s nothing to plan for. As far as humans are concerned, global thermonuclear war would be preferable. Everyone not already nearly on their deathbed would be affected.
I read the scenario as where unprotected sex used to have a 50% chance (made up number) of resulting in pregnancy, it now has a 5% chance.
Any scenario where 90% of either or both sexes becomes infertile can be worked around (at least to an extent) but forcing those people to reproduce to the maximum extent.
If the birth rate dropped to 10% of normal tomorrow, we’d have fifteen years before it would even start to show up in labor statistics, and decades longer before we get to the point where our work force is reduced to 10% of the current size. That’s a lot of time to start making changes to society to handle the new population levels, including making a whole bunch of robots.
There’d be a substantial impact on Social Security (in the US), since the current model is a “pay as you go” which relies on younger workers’ earnings to fund retirees. Benefits would be impacted - likely a higher “full retirement age”. People would be encouraged to work longer, both to reduce the draw on SS, and also to prolong their own contributions.
There’d less incentive to force out aging workers. since they would not have replacements.
Abortion would become even more criminalized than it already has. “Unwanted pregnancy” would likely become a thing of the past; any woman who didn’t want to have a baby would have massive incentive to continue the pregnancy - likely adoption-for-money would become a very real thing.
In the movie Children of Men, a LOT of people had dogs; pet ownership would certainly increase.
Also, depending on the cause of the reduction, people who retained their fertility would potentially pass that ability on to their children. After a generation, all the young adults would be breeders, and would be able to have many children if they desired. I could see the issue basically resetting human population to, say, 100 years ago, but eventually it would catch back up.