The cakeday plugin isn’t bundled in 3.5.4, so that’ll be why you’re not seeing it any more.
Are you sure that’s why they started failing? If you saw something like:
HINT: The plugin ‘$plugin’ is now bundled with Discourse and should not be included in your container configuration
then I’m afraid that will be shown for any failed rebuild when the cakeday plugin is included in the config file. This was the most efficient way we could warn people about the issue, but it can be confusing for people running older core versions.
So if you need the cakeday plugin, I think you should be able to re-add it to your app.yml file and rebuild. I suspect the failure was caused by something else, which you’ve now resolved.
As an aside: I’d strongly recommend upgrading to a supported version. 3.5 is not receiving security updates any more, and may be vulnerable to attack.