I wonder can the kernel just remove it and distros put on a compatiability layer.

It's already a configurable option in the kernel which can be fully disabled by distros if they wanted to provide their own compatibility layer, or just not ship any software that has a hard dependency on it.

I always use only custom compiled kernels on my computers, where I enable only the configuration options that I really need.

So the options related to AF_ALG have always been disabled, because I have not encountered an application that needs them, among those that I use.

Unfortunately the Linux distributions must enable in their default configuration most options, because they cannot predict what their users will need.