Dataverse - How to switch to the classic designer when the option is not available
In recent weeks, there have been several updates to parts of the Dataverse designer which have disabled buttons and necessary features. For example, some users have experienced problems where the save button is always disabled, which makes it impossible to save changes.

How to access the Classic Designer by URL
To workaround this issue, we can attempt to open the classic designer directly using the web address. The section in the classic designer that provides most access to Dataverse objects is the default solution.
To access this section of the classic designer, we require the organisation ID and the ID of the default solution. The format of the address that we use looks like this:
https://orgfe0f8107.crm11.dynamics.com/tools/solution/edit.aspx?id=fd140aaf-4df4-11dd-bd17-0019b9312238
We can find our organisation ID by starting any model driven app, and inspecting the URL.

To find the ID the of the 'Default solution', we can navigate to Solutions > Default solution from the modern Power Apps portal. The ID appears at the end of the address.

We can now construct the web address and use it to open the default solution in classic designer. As the screenshot beneath shows, we can access all of the tables via the entities node (entities is the old name for tables), and use the navigation pane on the left hand panel to access other features that may be missing in the modern designer.

To find the ID the of the 'Default solution', we can navigate to Solutions > Default solution from the modern Power Apps portal. The ID appears at the end of the address.

We can now construct the web address and use it to open the default solution in classic designer. As the screenshot beneath shows, we can access all of the tables via the entities node (entities is the old name for tables), and use the navigation pane on the left hand panel to access other features that may be missing in the modern designer.

Conclusion
If some part of the modern designer doesn't work as expected (possibly following an update to the Power Platform) and the option to open the classic designer is missing, we can attempt to overcome this problem by navigating directly to the classic designer by constructing the URL that opens the default solution.