This!
I live in Ireland where non-competes are illegal under EU law, but that doesn't stop some employers trying their luck.
I was made redundant from a company that provided a SaaS retail reporting/analytics system. They had never added a mechanism or developed a business process to easily cut off customers who weren't paying their bills, so rather than fix it they made a few people redundant to make up for the shortfall.
A friend introduced me to a rival firm who liked me enough to hire me, this was during my 30 day notice period when I was still working for my old employer. I never told my old employer that I had been hired as I knew the MD would lose his shit. Unfortunately I told one person there whom I considered a friend. I swore him to secrecy, but he had a drinking problem - I think you can see where this is leading. The MD got wind that my friend knew where I was going and one day (after I had left, but before starting the new job) when he arrived in a bit hungover the MD said "Tell me where he's working or I'll fire you for being hungover". So he blabbed on me.
The next thing I know I'm having abusive texts and emails sent to me, a motorcycle courier delivered a letter ordering me to not start with the new company. He even went to the Garda and falsely claimed that I was stealing trade secrets - they told him to go see a solicitor.
Then he came banging on my door in a rage. I had to call the Guards on him.
Although the new employer initially supported me, after a while they said I had baggage and they had to withdraw the offer.
As for my old boss, I had to ask the Guards to get him to back off.
Yep, you old employer does not need to know where you are going.