IFFT of a complex, conjugate-symmetric transfer function - Value at Nyquist frequency
If you have an exactly conjugate-symmetric frequency domain vector, the values  at DC and at Nyquist must be real-valued. The 'symmetric' flag of Matlab's ifft command makes sure that the result of the inverse FFT is real-valued, implying that the values at DC and Nyquist must also be real-valued.

Let $X[k]$ be the length $N$ DFT of a real-valued sequence $x[n]$:

$$X[k]=\sum_{n=0}^{N-1}x[k]e^{-j2\pi nk/N}\tag{1}$$

The value at DC is

$$X[0]=\sum_{n=0}^{N-1}x[k]\tag{2}$$

which is clearly real-valued if $x[k]$ is real-valued. Similarly, assuming that $N$ is even, the value at Nyquist is

$$X[N/2]=\sum_{n=0}^{N-1}x[k]e^{-j\pi n}=\sum_{n=0}^{N-1}x[k](-1)^n\tag{3}$$

which is also real-valued for real-valued $x[n]$.

Conjugate symmetry means

$$X[k]=X^*[N-k]\tag{4}$$

which for $k=N/2$ (i.e., at Nyquist) gives

$$X[N/2]=X^*[N/2]\tag{5}$$

Of course, $(5)$ just shows in another way that the bin at Nyquist is real-valued for real-valued $x[k]$.