Can confirm Periwinkle is a thing. It started a couple of years ago.
However, as far as I know, it's also done. The scope was massively reduced once they realised the complexity of what they are trying to do (which isn't sunsetting Heroku, but more moving it in the direction that Salesforce Functions has taken). In the end, Periwinkle essentially transpired to be the banner that you now see at the top of Heroku properties.
Saying that, I've been out a few months. I don't know what the current plans might be, and I do wonder if recent events might have SFDC evaluating what's next.
I just evaluated SF functions and it didn't seem too great, especially for the $$$. Especially since one could setup an AWS lambda with the newer function url + IAM user auth, where AWS sig4 is already supported by Salesforce's Named credential system.
The codebase is sound. I would almost certainly expect that the reason for the slowness is the ability/diligence and paranoia levels the SFDC security teams have. They won't want to turn this back on until they are absolutely certain it's 100% again.
I disagree that you're bound. Most of the way you run your apps on Heroku works the same way anywhere, or can do with only the smallest amount of work. IMO if you want to migrate away, the biggest job is filling the gaps that Heroku provide for you, and don't realize they do until you don't have them anymore.
Can confirm Periwinkle is a thing. It started a couple of years ago.
However, as far as I know, it's also done. The scope was massively reduced once they realised the complexity of what they are trying to do (which isn't sunsetting Heroku, but more moving it in the direction that Salesforce Functions has taken). In the end, Periwinkle essentially transpired to be the banner that you now see at the top of Heroku properties.
Saying that, I've been out a few months. I don't know what the current plans might be, and I do wonder if recent events might have SFDC evaluating what's next.