this is from electrodynamics of moving bodies einstein's original SR paper.tell me when we defined a common time for A and B didnt we define lightspeed to be constant

Yes, in the early 1980s the meter was re-defined as the distance that light travels in 1/299792458 second, so the speed of light is now exactly 299792458 m/sec.

Before that happened, physicists had spent several decades measuring the speed of light with greater and greater precision. Eventually the biggest remaining uncertainty was in the old definition of the meter (the distance between two marks on a platinum-iridium bar in a basement somewhere in Paris). The definition of a second (based on the oscillation of an atomic transition) was more precise.

So we can still do experiments like the old "speed of light" experiments, except that now, instead of measuring the speed of light more and more precisely, they make the definition of the meter more and more precise.

Not quite. This section defines lightspeed to be isotropic (the same in all directions), not necessarily constant.

A point that can and should be made is that this same defintion of clock synchronization that makes the velocity of light isotropic makes the velocities of other particles isotropic if their energy or momentum specified . For example, the same clock synch method makes the speed of 1 Gev electrons isotropic as well as the speed of light - not surprising, really, since 1 Gev electons travel at essentially lightspeed (this may not have been known experimentally in Einstein's time but is well known nowdays).

The same clock synch defintion makes the velocity of electrons of other energies isotropic as well, even low energy ones. And it's not limited to electrons, any particle of a known mass and specified energy will have an isotropic velocity only when clocks are synchornized in the same manner that makes light isotropic.

In addition, momentum could also be specified as well as energy.

Thus the constancy of the speed of light is not the only reason for adopting Einstein's clock synchronization method.