I have never seen Gardening Leave used in this manner.

Usually (and I know this from personal experience) if you are senior enough you have a notice period of 3-6 months and under normal circumstances you are expected to do a proper handover before leaving. Sometimes they don't want you to come in to the office or have access to the corporate network/data but they still want access to you in case they need it.

Non-competes are usually 18-24 months and I don't think anyone is going to pay that for you to sit on your butt.

Two Sigma regularly pays people 12 - 24 months to do nothing after they leave in lieu of them taking an offer with anothing trading firm, if they were working in strategy or research roles.

Not in the US but in Sweden a NCA is enforceable only if the employee is being paid (up to 60% of the former salary) during the whole period of the NCA that can last up to a maximum of 18 months.

My understanding of gardening leave mostly comes from how Matt Levine talks about it: https://www.bloomberg.com/opinion/articles/2019-08-28/take-t...

How does it work if the former employee is on work visa? Do they still sign a no-show contract with the former employer?

The employee would just have a very long notice period - so they are officially employed under the same contract as before, but the employer might revoke their access and say they don't need to show up. (And of course the contract would prevent them working or consulting for another company without their employer's agreement during the notice period, just as it did before.)