Twisted Pair input routing on PCB
So, I guess the twisted pair inputs IN+ and IN- would also be 27MHz.
Am I correct is assuming so?

No, that's not correct. The bandwidth of the transmitted serialized data will be a little over \$N\$ x the bandwidth of each deserialized output. It's a little over \$N\$ times because the serializer adds synchronizing markers into the combined serialized data frame.
So, converting the problem to bits per second, if you combined 4 serial data streams of 1 bit per second, the serialized data will be 4 bits per second plus a little bit more (for the sync bits needed to decode the data). Obviously you are not running at 1 bit per second; I am just using that as an example.

In that case, what impedance should be maintained between the IN+ and
IN- twisted pair input signals to the deserializer? Should I maintain
the 100ohms impedance just like if it was a coax input or what should
I do?

It needs to match the transmission/characteristic impedance of the twisted pair interface cable.