Yes, this is the “new normal”.
VPC-based Lambda functions used to be removed immediately, but the ENI that had been (automatically) allocated so that it could communicate inside the VPC would be orphaned unless you had waiting a certain (unspecified) amount of time after the function had had traffic. This resulted in lots of orphaned ENIs, and it was easy to quickly reach the default soft-limit for ENIs in an account.
The recent change in behaviour (which I didn’t see mentioned anywhere officially, but had heard of others experiencing) means that your stack clean-up will take as long as it takes to clean up the associated resources (in this case the ENI).
I would hope that this gets faster in the future, but I don’t think AWS will commit to any timeline or durations (it’s just not their style).
Just to be clear, this has nothing to do with Serverless, and everything to do with VPC-based Lambda functions.