In some places, yes. South Korea is expected to shrink by 15M in the next 50 years, and to cut in half by 2100. Even with immediate drastic improvements in birth rate, it is expected to shrink significantly.

Don't populations typically oscillate sinusoidally?

What's that got to do with whether such steep declines are desirable?

Well that might impact the long term projections.

The frequency/period of the sinusoidal wave you're asking about can be very long. When the Roman Empire collapsed its collapse was in great part due to Rome's inability to reverse a steep, long, secular fertility decline. It took hundreds of years for Rome to go from growth to decline, then hundreds of years for the post-Roman Europe to get back to growth.

When a generation's female cohort is significantly smaller than the preceding generation's female cohort the increase in fertility needed to offset that is enormous relative to the fertility rate that led to the present generation's small female cohort. And the cultural and other reasons for the low fertility rate are difficult to change too. So the likelihood of reversal is very low, and the likelihood of continued female cohort decreases is high, therefore you have to think a severe low fertility patch will last at least two generations, and if the fertility rate right now is half the replacement rate then that bakes in a population reduction of 50% once the current non-fertile generations pass, so several generations down the line. But the risk of three or four generations of below-replacement fertility is pretty high, and that's how you get into multi-centennial periods. Nothing requires that we bounce back, either. We could go extinct just from refusal to reproduce.

Yes but the peaks of those oscillations can be extremely unpleasant.

Like everyone suddenly gets raped/killed while all the buildings are burnt down kinds of unpleasant.

Negative population, no. The minimum is zero.

Population declines, absolutely -- those are baked into the pie now, and they will be quite steep.