You asked – they listened. Sage v28.1 now features a read only user.
In Settings->User Management-> Users you can now choose to create a read only user. A read only user can still have remote access enabled and you can still control what parts of the program you want them to see.
A read only user cannot add, change or delete any data.
How will you know that you are logged in as a read only user?
Secondly if you try to enter any new data, you will receive the above error message.