Is it now normal practice not to receive a response after interview?

It isn't normal practice and it's not at all polite.

However, people are only human and life is busy. It is easy, once the decision is made, to move on and be distracted by other things. More than once I've found myself - a week after interviews - asking a manager if they'd given the unsuccessful candidates feedback only to be told "Oh, I thought you'd done that".

Difficult conversations are difficult and, as Andre implies, if the selection criteria aren't well-founded in data but based, rather, on those false friends "gut" and "instinct", it leaves managers with little to say in terms of feedback which they therefore decide not to give (or, rather, don't decide to give).

I encourage managers to use a simple scoring method so they can see at a glance in which areas a candidate was weak or, simply, not as strong as other candidates. But you can take a horse to water...

That said,

Many candidates, if they really wanted the job, write back threatening discrimination if they don’t reverse their decision

I really don't think this is true. Unless your interviewer is exceptionally blunt and offensive, and unless your selection criteria are really blatantly discriminatory it is surely gross exaggeration to suggest that "many" candidates do this. A tiny handful of candidates, perhaps. A justifiably troublesome bunch, to be sure, but a bunch who are better responded to with more transparency and more intelligent selection criteria rather than a frankly suspicious wall of silence.

It isn't normal practice and it's not at all polite.

However, people are only human and life is busy. It is easy, once the decision is made, to move on and be distracted by other things. More than once I've found myself - a week after interviews - asking a manager if they'd given the unsuccessful candidates feedback only to be told "Oh, I thought you'd done that".

Difficult conversations are difficult and, as Andre implies, if the selection criteria aren't well-founded in data but based, rather, on those false friends "gut" and "instinct", it leaves managers with little to say in terms of feedback which they therefore decide not to give (or, rather, don't decide to give).

I encourage managers to use a simple scoring method so they can see at a glance in which areas a candidate was weak or, simply, not as strong as other candidates. But you can take a horse to water...

That said,

Many candidates, if they really wanted the job, write back threatening discrimination if they don’t reverse their decision

I really don't think this is true. Unless your interviewer is exceptionally blunt and offensive, and unless your selection criteria are really blatantly discriminatory it is surely gross exaggeration to suggest that "many" candidates do this. A tiny handful of candidates, perhaps. A justifiably troublesome bunch, to be sure, but a bunch who are better responded to with more transparency and more intelligent selection criteria rather than a frankly suspicious wall of silence.

Some candidates may do it if they have been out of work for some time, it is impacting on their marriage, they have a family to support, are in debt, can't pay their mortgage and may even lose their house as a result of spending time and money attending interviews but still not getting a job.