@jeremywoakes
First I recommend strongly change the name of the column to OpenClosed in Excel File, delete and recreate the data source. There may be a problem if having the Slash / in column name.
Then, try something like this and it should work:
1) Place a data table on the canvas to test it. Suppose it is called DataTable2 for this example.
1b) Select a couple fields so you can see the data table columns - to do this click on "Edit Fields" on the right side, and select some fields you want - such as OpenClosed field and Client field for example. If having trouble, go to the tree view, select DataTable2 first from the tree view, then try again from the right side.
2) In Items Property of the data table (suppose it is called DataTable2 for this example) put this:
Filter(Table1,OpenClosed=VarOpenClosed)
3) Place a Toggle property on the canvas. Suppose it is called Toggle1 for this example.
4) (Optional) Regarding Toggle1 - > for the Properties:
TrueText
put "Open"
and
FalseText
put "Closed"
5) For Screen1 (the first screen) OnVisible property (the OnVisible property of Screen1) put this:
Set(VarOpenClosed,"Open")
6) For the Default property of Toggle1 put
true
7) For OnChange property of Toggle1
put
If(Self.Value=true,Set(VarOpenClosed,"Open"),Set(VarOpenClosed,"Closed"))
😎 😎 Try the app.

I have also attached a working sample msapp file pl2_8-11-2021.msapp if you would like to import a full, working example yourself for your convenience.
To use the sample msapp attached, follow these steps:
1) Download the msapp file attached to this post, to Desktop or a folder of your choice, by clicking on it from this post.
2) Create a new, blank Power App Canvas App
3) Go to File -> Open -> Browse

4) Navigate to location of .msapp file from Step 1, select it, and press the "Open" button.
5) The working example msapp file should load
6) Note, the msapp file will come in without the connector (intentionally). You'll need to set up the Excel Online (business) connector and hook it to your sample excel file to see the sample app I made fully working
7) After step 6, you can try previewing the app and using the Toggle, and it should show Open cases only, or Closed cases only.
Check if it helps.