Distributor ID: Debian
Description: Debian GNU/Linux 10 (buster)
Release: 10
Codename: buster
x86_64
Linux 4.19.0-17-amd64 #1 SMP Debian 4.19.194-3 (2021-07-18) x86_64 GNU/Linux

Systemd units, sockets and services, with @ in the name are multi-instance systemd services. They are started with mariadb@name.socket and mariadb-extra@name.socket.

They are not installed enabled https://github.com/MariaDB/server/blob/10.6/debian/rules#L185-L191 and shouldn't be enabled or started without the name. If you have these enabled try to disable with systemctl disable mariadb-extra@.socket

Starting/stopping mariadb.service should have no dependency to the multi-instance names.

docs https://mariadb.com/kb/en/systemd/#multi-instance-socket-activation

Daniel, thanks for trying to help. I'll check if I can turn them off.
But sockets were installed off. I found mentions of this in the /var/log/apt/term.log:

New version of the configuration file is installed  /etc/mysql/mariadb.conf.d/50-server.cnf …

mariadb-extra.socket is a disabled or a static unit, not starting it.

mariadb-extra.socket is a disabled or a static unit, not starting it.

And another moment. A similar error appears on 2 machines - remote VDS and Debian 10 on a Windows 10 Hyper-V virtual machine.
This means that this behavior of mariadb is typical ...

As I assumed, disabling the sockets did nothing. It's not that they are running. They have been disabled since installation.
But why does systemctl check mariadb sockets when stopping any service?

I don't know. I checked the systemd source code and this message not longer is part of it.

Daniel, thank you. I will wait for a response from a person who understands what the problem is. Or he knows how to solve it.

I just don't understand why you closed my question? I would not be so disgraced if I were you ...
Don't know the answer and make a smart face? I'm sorry that I told you all these compliments - you are nothing. Just 0. Empty space.

You got a harmess, though slightly confusing message, that isn't generated by MariaDB, that we can't change. I'm not smirking on this. I just don't see a problem to be solved. And there are other bugs that crash and cause people real trouble.

Developer ... Developers don't say "I don't know." Empty space ... And we still use what these "I don't know."

Just put on your glasses. You are blind. I will solve this problem myself. Without you - the developers ...
It was not pleasant to talk! Good luck.

Please see MDEV-27715 if it's working for you and feel free to contribute if it's not fixing problem.