Although cl_interp has been left in on console, it is now inactive and should have been removed, alfred from valve has mentioned on this.
Also the interp calculations have been gone over many many time and depending on you interp ratio, and your updaterate, below are the value of cl_interp you can achieve.
Its much better than the old system, with the max cl_interp with default rates coming out at 0.067, and as soon as we go past updaterate of 60, the communitys lowest agreeable, we hit a highest possible interp of 0.03
cl_interp_ratio simply causes the interpolation delay to be calculated off the clients cl_updaterate (The amount of updates the clients receive per second. This will not exceed the servers sv_maxupdaterate or servers tickrate, which ever is the smaller)
The outcome for this is as follows:
cl_interp_ratio 1.0 cl_updaterate 30 interpolation = 0.033
cl_interp_ratio 1.0 cl_updaterate 35 interpolation = 0.029
cl_interp_ratio 1.0 cl_updaterate 40 interpolation = 0.025
cl_interp_ratio 1.0 cl_updaterate 50 interpolation = 0.020
cl_interp_ratio 1.0 cl_updaterate 60 interpolation = 0.017
cl_interp_ratio 1.0 cl_updaterate 66 interpolation = 0.015
cl_interp_ratio 1.0 cl_updaterate 75 interpolation = 0.013
cl_interp_ratio 1.0 cl_updaterate 80 interpolation = 0.013
cl_interp_ratio 1.0 cl_updaterate 100 interpolation = 0.010
cl_interp_ratio 1.5 cl_updaterate 30 interpolation = 0.050
cl_interp_ratio 1.5 cl_updaterate 35 interpolation = 0.043
cl_interp_ratio 1.5 cl_updaterate 40 interpolation = 0.038
cl_interp_ratio 1.5 cl_updaterate 50 interpolation = 0.030
cl_interp_ratio 1.5 cl_updaterate 60 interpolation = 0.025
cl_interp_ratio 1.5 cl_updaterate 66 interpolation = 0.023
cl_interp_ratio 1.5 cl_updaterate 75 interpolation = 0.020
cl_interp_ratio 1.5 cl_updaterate 80 interpolation = 0.019
cl_interp_ratio 1.5 cl_updaterate 100 interpolation = 0.015
cl_interp_ratio 2.0 cl_updaterate 30 interpolation = 0.067
cl_interp_ratio 2.0 cl_updaterate 35 interpolation = 0.057
cl_interp_ratio 2.0 cl_updaterate 40 interpolation = 0.050
cl_interp_ratio 2.0 cl_updaterate 50 interpolation = 0.040
cl_interp_ratio 2.0 cl_updaterate 60 interpolation = 0.033
cl_interp_ratio 2.0 cl_updaterate 66 interpolation = 0.030
cl_interp_ratio 2.0 cl_updaterate 75 interpolation = 0.027
cl_interp_ratio 2.0 cl_updaterate 80 interpolation = 0.025
cl_interp_ratio 2.0 cl_updaterate 100 interpolation = 0.020
Clients can have higher cl_interp_ratio values to accomodate for packetloss & choke.
There are 2 server side variables that limit how large a clients cl_interp_ratio can be, these are:
"sv_client_min_interp_ratio" = "1" replicated - This can be used to limit the value of cl_interp_ratio for connected clients (only while they are connected). -1 = let clients set cl_interp_ratio to anything any other value = set minimum value for cl_interp_ratio
"sv_client_max_interp_ratio" = "2" replicated - This can be used to limit the value of cl_interp_ratio for connected clients (only while they are connected). If sv_client_min_interp_ratio is -1, then this cvar has no effect.
The bottom line is this: interp is calculated off your updaterate. Change your updaterate and your interp will change, it is as simple as that!