BCS External Lists are Failing

[BandDad]

BCS External Lists (connected to SQL Server VM on Azure) that had been working for more than 4 years have just stopped working out of the blue.  I get the dreaded:

Unable to display this Web Part.  ...

With a correlation ID.

[#2]

Open a support ticket...Microsoft has not done any work in BCS for years and I suspect they are doing / considering to do changes in the backend that might affect legacy services such as BCS. For instance, we learnt this week that SP 2010 workflows in SPO are going to be deprecated starting next month

[#3]

Thanks for the response @Juan Carlos González Martín and I agree with your suggestion that legacy features are getting pruned.

A trouble ticket was opened and here was the response:

"To recap, to trouble shoot this issue SharePoint designers is needed,  However SharePoint Designer is not covered under MSFT 365 support."

My guess is that some new rules are being enforced on exactly what SQL sources you can connect to.

Per this article 

https://support.microsoft.com/en-us/office/introduction-to-external-data-676e60e7-d99f-463f-a173-65e...
"SPO only supports WCF and SQL Server Azure Connectors"

It appears that the SharePoint Online team just started enforcing this in the last few days (July 4th or 5th, 2020).  SQL Virtual Machines apparently did not make the cut.

-Ben

[#4]

@BandDad 

I have also been looking through the same documentation.  In my case I have been asked surface 

external lists hosted in classic SPO pages ( yes really!)  and with data residing on SQL Server 2016 on-premises, via the Azure Data Gateway.  I am trying to get my head around what can be achieved or indeed supported

1.  External lists  without BCS, 
2. External  lists using WCF, with BCS configured on SharePoint Online 
3. Only available to users with E3 or E5 or SharePoint Plan 2 licenses
4. Creating DataSources in 1) SharePoint Designer 2010 2) SharePoint 2013.  
5. Just going with SQL Server Connectors via Power Apps designer - this does work yay!