Some people say mixing old coffee grounds into garden soil makes it
better for growing plants.

Should be used with great care, Coffee plants exclude a substances
that inhibits seedling growth, this is their way of making sure their
seedlings have a better chance of survival
James Wong of GQT did a number of experiments a few years ago and
proved that coffee grounds are not good for the soil

A few handfuls mixed in with the compost heap will not be a problem,
but a sack full for the local coffee shop would be

Last week's GQT had this as its second question,

As for it stops slugs, plenty of videos showing that it does not stop
them, easy to see why, slugs travel on the mucus they produce, so
nothing stops them accept a water barrier, (and even then they may go
under it)