This is devastating news for us 
It definitely erodes the trust we have in using the Atlassian platform for our apps.
I guess this was already hinted at when Atlassian tried to rename the External Asset Platform to “Legacy Assets”. At the time, it was communicated to us that there are no plans to deprecate the External Asset Platform. Less than a year later, this is exactly what’s happening now.
It affects three of our most successful apps. Using JSM Assets is not an option, since most of our customers of those apps are not using JSM premium. It’s not a realistic option for us and our customers. Migrating to Forge is also not an option, since all of those apps are Connect apps and it’s not yet possible to migrate our Connect apps to Forge.
All I can ask is to delay the deprecation until we can utilize Forge extension points in our existing Connect apps, so we can replicate the External Asset Platform in our apps - whenever that will be.