Please tell me how to return the old version of os class c 8.0.1 to 3.8 I need the original version and there are too many published ads on the site. in order to install it from the beginning

Your request does not make too much sense.
Why would you revert to 3.8 and then reinstall?

It is also not secure to stay on outdated version.
Anyway, you can open upgrade-func.php and revert changes from bottom to top.
You must do those that change table structure or create new table, those entries that just create new parameters can stay...

I had your osclass 4.4, there were no problems with plugins from other developers. when I switched to version 8.0.0. “slider round” stopped working in the “SEO PRO Plugin” plugin from the developers of “osclass evolution” I wrote them about an error, they said that this problem is in your version. they refused to help me, which made me very upset Help bring back the old version

Osclass evolution is dead, their forums is dead, development is dead, market is dead so I do not quite see reason why they would not check the issue.
You can PM me login to your backoffice and will review issue. I suppose it could be related to different jQuery version as backoffice already use latest one.
You could also change jquery version change it Settings > General, but I am not sure if this affect backoffice as well.

I am very grateful to you for helping me. thanks a lot !
Can you please tell me where can I send you my username and password?

@Дима Пупкин
This should make it, add to i.e. admin.css of plugin or new.css in omega theme.
.switch {z-index:9;}
... does not look like to be big issue 

We will add it in osclass 8.0.2 as well as bit more optimizations.

Thanks a lot! everything is working

coo