There exist tags for totient-function and for divisor-sum.

If these two arithmetical functions deserve to have their own tag, I would say that Möbius function should have a separate tag as well.

I do not see such tag anywhere, I was only able to find moebius-inversion.

There is also a relatively new tag (only one question) divisor-counting-function.

It was probably created here.

@Lord_Farin I am not sure whether you follow this room. I have bumped the meta question about congruences tag. I think it would be nice to have this finally resolved somehow.