When Did you stop and start to play again? Interested about your thoughts regarding rule and material changes.
I stopped more or less in '84, had a hard time quitting even though injured and in the end got disgusted. I tried to train seriously every now and then, but was hampered and restricted in movement right at the point that I felt I really needed to put in the hours to cope with growing stronger, taller, and less agile. My peers developed, and I got deeply frustrated.
This was already after the two-color rule got introduced of course. And the tightening of the service rules. The two color rule forced me to change rubbers right at the time when I was gaining strength and learning to maximize FH power transfer, and that's when my injury started. I have my reasons for disliking that rule.
In the meanwhile I had a very short bout while studying, about 10 years later at a club in a college town. The atmosphere of that club was unfriendly to "outsiders", just regulars playing the odd game as an excuse for beer and cardplay throughout the evening. Tried it for a few months then turned away in renewed disgust again, not being able to make any headway there.
I started to play again late 2015 or early 2016, having moved to a smaller town. Life had happened inbetween, and I had raised two children. When I started suspecting they had grown up enough to survive a few hours without me in the evenings, I checked out a local club in a nearby town. This time I found a welcome environment, with a few people eager to play. (I now chair that club. So it goes.)
That was right about the time the celluloid 40mm ball was replaced by plastic ones. I haven't played much with 40mm celluloid for that reason. I did find the "new game" different, and I missed the spin level and the greater penetrative power of the 38mm ball. The first year I played, I found the overall quality of the balls very poor. A lot of "eggs", irregular bounces, and I've had several balls just fall apart by hitting them at that time.
After a while that improved; ball quality improved, what we're playing with now is predictable and consistent. And I've come like some of the effects the ball has on the game; in general, it seems to me a typical rally often takes one decisive stroke extra, after the one that would have sealed it before. It keeps me hanging in there, and constantly reminds me of the need to reset-to-position. In some senses I'm a more powerful and complete player than in peak youth form, but of course age brings its limits.
What I found hard to adapt to (and still find) is the service rule. I've been so deeply trained in types of deception that are now "illegal" that consistently serving with good quality in an acceptable fashion remains challenging.
Another thing that took a while, probably related to the 40mm ball, is the hugely increased speed and elasticity/explosiveness of the "modern" rubbers, i.e. the ones from the last two decades or so that pretty much all attackers use. Way back when I started out with Mark V on an Offensive Wood blade, and got scolded by my coach because that was considered crazy fast. That same setup would now be seen as a slow and dull allround one.
It took me a year or so to get my technique recalibrated, to hit with full consistency again. Footwork and game smarts take a bit longer.
Another change, in the meantime, is that after I quit the backhand got activated more and more. That trend was already starting, early 80s, but still, the BH kept the ball in play, the FH opened it up and killed it. We flicked on both wings, and spinned on both wings, but not like we're doing now. Being prepared to play a second-ball countering game against aggressive service openers was a new dimension for me.