We have removed support for the TAGS command line option as part of the TFLM Makefile. The reasoning behind that was to avoid a catch-all list that introduced additional complexity to the Makefiles and made error checking very hard.
For the majority of the cases, TAGS=<optimized kernel implementation> should be replaced with OPTIMIZED_KERNEL_DIR=<optimized kernel implementation> and things should continue to work as before.
If you were using TAGS for anything else, your workflow will be broken. Please file a github issue and tag me (@advaitjain) so that I can help resolve your issue.
One of the benefits of the changes that we have made (besides being easier to maintain) is that having an incorrect OPTIMIZED_KERNEL_DIR will give you an error whereas an incorrect TAG would have been silently ignored and your application would fall back to the reference kernels.