I have no opinion on how it may have been in the past, since I've only been actively thrifting for audio equipment around 1 year now. Most of the time I find BPC, lightweight all-in-one rack systems, plastic boomboxes, tiny computer speakers, miscellaneous remote controls, DVD players, small iPod docks and 'tower' speakers that aren't even 8" deep and look to be made of pressboard that's about 1/4" thick.
I keep going consistently though. 95% of the time there isn't anything of interest and I'm in & out in about 5-10 minutes. 4.5% of the time I find perhaps a couple of CDs, DVDs, records, tools, or other non audio-related stuff that I need/want for the house or my other hobbies. I stop in 2-3 times a week because of that 0.05% when I find something incredible. In one year I've picked up two silver faced monster receivers, 4-5 other good quality silver faced amps/receivers, 5-6 good quality turntables, a full stack of separates, 5 or so sets of quality, brand-name tower speakers, a few sets of brand-name bookshelves, some good home theatre speaker sets, working PA and professional sound reinforcement equipment, a number of working home theatre receivers, equalizers, sound shapers, accessories (remotes, speaker stands, speaker switches, name brand shielded RCA cables, speaker cable, wall plates and banana jacks still in their packaging), and more. Whenever we visit family, or are in another town, we stop in at the thrift stores there too. I've filled the trunk with some pretty decent stuff while on vacation.
I don't have a data plan on my smartphone, so I have developed an eye for the indicators of quality workmanship. There are simply too many esoteric brand names to try and memorize them all, or keep a list of every single desirable thing there is out there. I do agree that yard and estate sales are incredible resources as well. Within two or three weeks of each other I found a functioning tube powered extension cabinet at an estate sale, and then another from a different manufacturer in a thrift store. Haven't seen another one before, or since. You can not predict when incredible things will wind up sitting on the grass on a Saturday morning, any less than you can predict when they will hit the shelf at the thrift store, or the bottom of a dumpster. I've seen stuff disappear as soon as the doors open, and I've also found extremely desirable items by popping in at 5 minutes to closing time. It comes down to luck and consistency, at least that's been my experience since June of last year when I began this crazy ride.
Online classified ads are where my region is a major letdown. Other, larger areas than mine might have plenty of deals showing up frequently, but I basically never see anything within 200 miles of me. I saw no deals whatsoever while looking daily for 12 months, until my recent score of the year that is, and that was down to luck with timing I think. Geographic and socioeconomic factors probably play a decent part in what is available in each area, and which distribution channel produces more of the goodies.