All-zeros subnet and All-ones subnet It is said that practically all-zeros and all-ones subnets are not used.

Let us consider an a network $200.200.13.13/25$

Here Network ID is represented by using $25$ bits and Host ID using $32-25=7$ bits.

Binary Representation : $11001000.11001000.00001101.00000000$

Last 7 bits which represent HID are $0000000$

Suppose we want to divide the network into $4$ subnets, so we will borrow $2$ bits from Host.

So the subnets will be

$11001000.11001000.00001101.00000000$ - $200.200.0.0/27$

$11001000.11001000.00001101.01000000$ - $200.200.0.64/27$

$11001000.11001000.00001101.10000000$ - $200.200.0.128/27$

$11001000.11001000.00001101.11000000$ - $200.200.0.192/27$

Now it is said that all-zeros and all-ones subnets are not used, then let us consider -

(i) the IP addresses ranging from $200.200.0.0 - 200.200.0.63$ which should belong to all-zeros subnet and

(ii) the IP addresses ranging from $200.200.0.192 - 200.200.0.255$ which should belong to all-ones subnet

Will belong to which subnet/network or will remain unallocated ?
Not "it is said", in IPv4 the "host part all zeros" address defines the (sub)net (and isn't available as host address), the "host all ones" address is broadcast for the (sub)net (and also isn't available as host address). All other addresses can certainly be used (and given that IPv4 addresses ran out, there is a lot of pressure to use all).