In the file sogo-integrator-31.0.0-sogo-demo.xpi\chrome\content\calendar\folder-handler.js must add at line 115 "directory.setProperty (" readOnly ", true);"

This helps when subscribing to read only by default.

This can be useful in a business

This is still a very nasty issue (basically the same as 0002137). Manually setting calendar.registry.<hash>.ReadOnly to True fixes the issue.
There is no GUI to set that flag and loading ACLs for a calendar also does not fix that.
Are there any plans to fix that?

you just add in plugin sogo integrator at line 115 of file \chrome\content\calendar\folder-handler.js "directory.setProperty (" readOnly ", true);"

add this line to activate readonly on a calendar or not by checking if the user has write access to the shared calendar .

The suggested fix does indeed set proper calendar permissions for newly subscribed calendars or on new setups.
There are no changes on already existing calendars; refreshing ACLs on a calendar also does not change this.