For example:
API 16-20 : 4 split by ABI APKs
API 21+ : appbundle
I need this, because I don't support 16-20 anymore and use old release for them. https://developer.android.com/google/play/publishing/multiple-apks#HowItWorks
I know, I can try it myself, but I'm unable to test this right now.
Yes, it's possible to have a mix of APKs and App Bundles in the same release.
If I understand correctly, you want to upload App Bundles for new versions, but carry over APKs from previous releases for devices that you no longer support. That is a common use-case and is indeed supported.