AFAIK, world population is projected to peak around 11 billion, which by many estimates could be feasibly sustainable with a few technological improvements to increase crop yields and decrease carbon emissions.
The current population is not even sustainable. Half it and then perhaps it will be sustainable, if capped. Ecological damage will probably increase in the future, as swathes of people from developing countries will want to have the same living standards, that the first world countries experienced for decades. If you combine that with 11 billion people, it will be a disaster.
I recently heard about China stopping the one-child law stuff and I only wanted to facepalm. That's so typical. "We need more population for all our old people here!" disregarding what will happen with the planet (an objectively much more important thing than China, but hey, it's not China, so not their problem, right?!), if population surges again in China. I just hope, that the experts are correct and that it will not lead to another population increase.
https://ourworldindata.org/fertility-rate#what-explains-the-...
If you make a certain set of highly unrealistic assumptions about what "sustainable" requires, yes.
If you look at how human history has actually gone, you find that doomsayers in every time could find trends that, if projected out forever into the future, looked "unsustainable". And of course, if those trends had continued as they were, it would not have been sustainable. But the trends never continued as they were.
We humans can adapt to change by changing how we do things. Claims that our current population, or indeed any current human activity, is "unsustainable" ignore that crucial fact.
11 billion people living in the same way we are living in the first world would be impossible.
If you ask me if I would trade places with that 200 years ago person I would say no. If you ask a person 200 years hence if they'd trade places with me I have no doubt that they'd say no as well.