Sure, common practice "back when", mentioned here often in your context. Used only with vacuum tube amplifiers, of course.
Be careful since the ratings of home stereo equipment are not necessarily consistent with industrial ratings. As a general rule, commercial stuff is specified as "peak" power rating (because recorded music power levels are of intermittent power levels) and should be de-rated by at least 50%. If you're looking at an L-pad rated at fifty watts, it would be useful for a twenty watt amplifier, for example.
L-pads have been VERY useful, to me, between the output of small amplifiers and the loudspeaker (with an SM-57 in front of the loudspeaker connected to the PA system). Obviously the L-pad impedance should be the same as your loudspeaker impedance. I wouldn't recommend an L-pad with high power amplifiers - those using dual or quad speaker configurations.
The "power soaks" and similar products (there are many) are silly. Why lug around a 50 watt vacuum tube amplifier and generate a lot of waste heat to make it sound like an 18 watt vacuum tube amplifier? Sound reinforcement systems are so powerful and so inexpensive that there should be no reason for a guitarist to carry much more than a Deluxe Reverb or Ampeg Reverberocket + SM-57 + small stand.
Better still, a small but powerful solid-state device - even less expensive than all of the above

unless you're going for distortion.
Cheers !