Newcastle United absolutely can keep him on gardening leave for the remainder of his contract.
Sort of. As I've stated previously, it's typical for garden leave to match an employee's notice period. For the most senior employees - i.e. director level/SLT members such as Ashworth, that's typically a notice period of around 6-9 months at the heaviest. It's unusual for a period of garden leave to exceed these periods. This
may be drafted in the terms of his employment contract, but there's a strong chance it does not fly in practice. Ashworth could very easily take this to the employment tribunal and argue that the terms of his garden leave are unreasonable and greatly exceed their necessity -- (20 months of doing nothing will massively hamper his job opportunities going forward).
Can Newcastle place him on garden leave, prevent him from accessing any confidential information, and completely freeze him out of the club? Yes. Can they practically do that for a period of 20 months? Unlikely.
What your contract says, and whether things are actually enforceable are two different things. Generally speaking, nearly all the terms in your employment contract will be enforceable. Restrictive covenants and garden leave are the main two that fall into the 'are they enforceable' bracket. It will all depend on how reasonable it is to enforce those in their entirety taking everything into consideration.
What will happen is the lawyers will thrash this out in the coming months and a settlement figure will be reached.
Correct.