Let's calculate a bit:
2.8 x 30 x 60 = 5040 Megabyte = 4.9GiB
2.8 x 40 x 60 = 6720 Megabyte = 6,6GiB

That is how much you download before the connection bandwidth drops to 40KiB/s
what else can you tell me and others about your hardware?

Do you use Mobile Internet (G4)? (which is intended for smartphones) Is it maybe the default in-one-go limit? (You'd need to look on your Data Plan provider's site if this is the case.)

What are you downloading to? An SSD? How full is it?

Are you doing anything with your computer while downloading?

I see Turkey, and have noticed this issue from more Turks. Possibly then with the same Turkish ISP. That is, I suspect your ISP may be throttling long-running Steam connects (or if you happen to be on an e.g. campus/apartment network that it may be).

I'd search the internet more widely for this specifically for your ISP / network.

Thanks for the response. Here’s some additional info:

1. Connection:
Using fixed broadband, not 4G. Speedtest shows 2.8MB/s, which I get initially on Steam, but it drops to 40KiB/s after 30-40 minutes.

2. Hardware:
Downloading to an SSD with 35% free space. No other resource-heavy tasks are running during the download.

3. Other Platforms:
No issues when downloading from other platforms.

Could this be a Steam setting, or something with my ISP?

Yeah, I think it could be your ISP or maybe your router if you have one.
https://gtm.you1.cn/helpsteam/en/faqs/view/5AC5-8056-E88F-F3FF
It seems kinda intentional, a sudden drop to 40kb/s after 6GiB download.

You might be able to bypass it with Cloudflare's WARP if it is the ISP. https://one.one.one.one/

You can also try look at your power settings, and whether or not Windows is allowed to "power down the device" if it thinks it is not being used as much; but other than this...
I have no clue.

The amount of gigabyte is too large for it to be a simple caching issue. In any case, I have absolutely no clue at the moment.

--
That said,try to optimize your internet. Change DNS, try another download server, use TCP Optimizer a bit, disable things like: sysmain, file indexing, power saving optimizations, write caching on the device (if it is an ssd), bitlocker, networkthrottling index, etc.

maybe it helps a it.

You could perhaps test what happens if you after a downloads drops pause the download, switch download regions and resume the download. If that also works to get you up at least from that 40 KiB/s again I'd say that it's indeed your ISP throttling a long-running Steam connection.

If it doesn't, and if you make sure that Steam was in fact using the new server after the switch/resume then I guess not.

EDIT: Crap, never mind. Just tested and a change of download region needs a client restart so that couldn't tell the difference.

Yet another Turk with the issue: https://gtm.you1.cn/discussions/forum/1/4423184732108973969/

引用自 Erman：

When I want to download a game on Steam and the download starts, I can download the game at the speed indicated by Speedtest (2.8MB/s) for about 30-40 minutes. However, then something happens and my speed suddenly drops to 40kb/s, making it impossible to download the entire game. The strange part is that if I close Steam and open it again, I can download at maximum speed (2.8 MB/s) again, but after 30-40 minutes, my speed drops to 40kb/s again.

I was able to download a game by repeatedly closing and reopening Steam. I don't experience this issue when downloading games from other platforms. Could you help me with this issue?

Hocam aynı şey banada yaşanıyor hatta adam aşşağıda benim postun linkini atmış internet sağlayıcın konumun felan nere bi ilgi alışverişi yapalım

引用自 Yujah：

Yet another Turk with the issue: https://gtm.you1.cn/discussions/forum/1/4423184732108973969/

Same issue with the same game lol. I am using Superonline Turkish ISP.

Probably safe to say then that, yes, it's something specific to your ISP. And thanks for specifying which: is going to be something to point others with the issue to.

引用自 Elucidator：

You might be able to bypass it with Cloudflare's WARP if it is the ISP. https://one.one.one.one/

I know I mentioned this tool before, but currently I think I will highly recommend it.
Here's what it does: https://developers.cloudflare.com/warp-client/get-started/windows/

Another person from Turkiye and using the same ISP has had a positive experience. They could download without the throttling. I recommend you to test it as well.