Is this a cold decision?

[xise]

To cut off people completely from your life if they are seen to be ''toxic'' or whatever? be it a family member or a friend you've been with for a decade

A few of them may not be happy with the departure at all.
I knew I will have second thoughts about this. because I'm a fool sometimes.

Have you ever done this? what are your thoughts?

Usually, I think it's more loving to think of it as a long break (and probably word it as such).

People can and do change over the course of years.

I think it's healthy to set boundaries and temporarily break contact (even for extended periods) if the person is on a path that contradicts yours. But it's good to be open to a different future. The finality of cutting one off from another permanently in these cases often seems to be intermixed with judgment, because we never can know how a person will use their free will to change in the distant future. 

That being said, I think in a non-ideal world of forcing to choose between continued contact or a permanent break, a permanent break can be healthier than continued contact with a currently toxic individual. Though I think a long break is best (can be years).