I don't understand how you get to http://mirror.mephi.ru/devuan/devuan/di … backports/ from your sources.list ... and what relevance that has here

I'm (also) using the same list:

deb http://deb.devuan.org/merged chimaera main contrib non-free
deb http://deb.devuan.org/merged chimaera-security main contrib non-free
deb http://deb.devuan.org/merged chimaera-updates main contrib non-free
deb http://archive.devuan.org/merged chimaera-backports main contrib non-free

and don't have any problem.

# apt-get update
Get:1 http://archive.devuan.org/merged chimaera-backports InRelease [26.5 kB]  
Get:2 http://archive.devuan.org/merged chimaera-backports/main all Packages [176 kB]
Get:3 http://archive.devuan.org/merged chimaera-backports/main amd64 Packages [408 kB]
Get:4 http://archive.devuan.org/merged chimaera-backports/main Translation-en [370 kB]
Get:5 http://deb.devuan.org/merged chimaera InRelease [33.5 kB]                
Get:6 http://deb.devuan.org/merged chimaera-security InRelease [26.6 kB]       
Get:7 http://deb.devuan.org/merged chimaera-updates InRelease [26.1 kB]        
Get:8 http://archive.devuan.org/merged chimaera-backports/contrib all Packages [2632 B]
Get:9 http://archive.devuan.org/merged chimaera-backports/contrib amd64 Packages [6216 B]
Get:10 http://archive.devuan.org/merged chimaera-backports/contrib Translation-en [6345 B]
Get:11 http://archive.devuan.org/merged chimaera-backports/non-free all Packages [4936 B]
Get:12 http://deb.devuan.org/merged chimaera/main amd64 Packages [8310 kB]     
Get:13 http://archive.devuan.org/merged chimaera-backports/non-free amd64 Packages [14.7 kB]
Get:14 http://archive.devuan.org/merged chimaera-backports/non-free Translation-en [32.2 kB]
Get:15 http://deb.devuan.org/merged chimaera/main Translation-en [6476 kB]     
Get:16 http://deb.devuan.org/merged chimaera/contrib amd64 Packages [50.7 kB]  
Get:17 http://deb.devuan.org/merged chimaera/contrib Translation-en [46.9 kB]  
Get:18 http://deb.devuan.org/merged chimaera/non-free amd64 Packages [98.3 kB] 
Get:19 http://deb.devuan.org/merged chimaera/non-free Translation-en [91.4 kB] 
Get:20 http://deb.devuan.org/merged chimaera-security/main amd64 Packages [392 kB]
Get:21 http://deb.devuan.org/merged chimaera-security/main Translation-en [281 kB]
Get:22 http://deb.devuan.org/merged chimaera-security/contrib amd64 Packages [2908 B]
Get:23 http://deb.devuan.org/merged chimaera-security/contrib Translation-en [2722 B]
Get:24 http://deb.devuan.org/merged chimaera-security/non-free amd64 Packages [1200 B]
Get:25 http://deb.devuan.org/merged chimaera-security/non-free Translation-en [1274 B]
Get:26 http://deb.devuan.org/merged chimaera-updates/main amd64 Packages [17.9 kB]
Get:27 http://deb.devuan.org/merged chimaera-updates/main Translation-en [10.8 kB]
Fetched 16.9 MB in 23s (731 kB/s)                                              
Reading package lists... Done

Perhaps remove all files in /var/lib/apt/lists/ and then try apt-get update again.

I'm trying to setup a test; would you mind show your sources.list, as well as the apt configurations (in /etc/apt/apt.conf  /etc/apt/apt.conf.d/*)?

It might look odd, yes; there is Translation-en.bz2 in devuan while there is Translation-en.xz in debian. However that's the same for other (non-archived) codenames as well.

Do you get a complaint with apt-get update?

Yes.
Print it out, frame it and hang up on the wall.
Then you can show your kids and grand kids how adults do things.

It looks like there you have a task to volunteer for.

I wouldn't worry about that until it fills up; then removing old kernels is the first step. Though generally it may also be a good idea to keep a "live installer" USB stick on the shelf.

I'm not sure what that issue is about; my excalibur boot directory is 100 Mb, though also not a separate partition. I think 487 Mb is plenty and it's certainly fine as a raw partition.

EDIT: I also have an EFI partition with <5 Mb bootloader (grub), which gets mounted as /boot/efi.

Yes, it should be fine like that. Of course any dkms modules might play up.

I've also seen that some people have issues with their windowing programs with excalibur, but that's more of a post-upgrade concern.

Seems your DNS is playing up. First try ping -n deb.devuan.org and second try traceroute -n deb.devuan.org. Does that give anything?

Btw. use code blocks for code (and console output); not quote blocks.

It appears your system in the chroot has a directory "/run/systemd/system"; perhaps that chroot filesystem has a bind-mounted "/run" from an outer "grml live iso" system with that badness in it?

Yes, @tux_99, I read your post too hastily, and I thank you for your suggestion. My note about childish noise obviously doesn't apply to your offer.

The Devuan git store has had address block blocks a long time, and it does work to some extent. But as you would know, it's a method that requires constant monitoring and tuning, and that one can accept longish periods of seriously degraded service.

DDoS is an abbreviation for "Distributed Denial of Service". It is a label given to what happens when a "rouge actor" sets up a system where a large number of computers "hammers" a service with incessant networking, and the service gets bogged down so that it fails to provide service.

That is what git.devuan.org is suffering of; a constant such hammering is happening that is a magnitude or more larger than how it was just a year ago.

The Anubis front-end feature makes it possible for git.devuan.org to provide service.

Your input is without value unless you can provide an alternative solution.