public class SlowRequestController : Controller
{
    private readonly IRepository _repository;

    public SlowRequestController(IRepository repository)
    {
        _repository = repository;
    }

    [HttpGet("/slowtest")]
    public async Task<IActionResult> Get(CancellationToken cancellationToken)
    {
        await _repository.LongRunningTask(cancellationToken);

        return Ok();
    }
}

public class Repository : IRepository
{
    public async Task LongRunningTask(CancellationToken cancellationToken)
    {

        // token.ThrowIfCancellationRequested(); or below
        if (token.IsCancellationRequested) // true if RequestAborted by any reason
        {
            Console.WriteLine("Task cancelled");
            return;
        }

        // slow cancellable work
        await Task.Delay(10_000, cancellationToken);
    }
}