Most skills have a shelf-life. There is probably a ratio of time versus skill loss for most things just like physiological decline after physical training stops that can be graphed. For tennis, I'd put it at 5-10% per week down to 60%, and then it declines at a slower rate. Just a guess. Somebody has probably done a study.
If you find a study I'd love to see it.
There are many good reasons to learn a one-handed slice for a backhand. Most involve the simple physical benefit of extra reach.
I learned it just to solve the problem of reaching balls that were seemingly out of reach on the backhand. It is also very energy efficient when you're losing steam. Rafa does it now to save the running, extend his reach, and save an unnecessary deep knee bend on a short or wide low ball versus using a two-hander.
Learn slice early and you'll find the flexibility of having it available will make you a much more challenging opponent. Additionally, the more spin control you develop the more likely you'll manage it better on your serve, volleys, lobs, and drop shots.
In the USA we're a country with free markets and I am getting the feeling that what some want is to have an agency policing who gets to teach.
Lots of rec programs relied on high school kids on the tennis team to teach. I was one of them back in the '70s. We had no certifications but introduced the kids to a sport we loved and made it fun.
The same was what happened to me. My first instructor was a $5 Department of Recreation five-week class with a high school kid teaching it back in the 1960s. He was good enough and I have great memories of it now 58 years later.
Now, many rec programs want certified instructors but the majority rely still on those less qualified than a private club would require. You don't need a high-level coach introducing kids to tennis but rather someone who is motivated to share the game and give them reasons to want to come back for more.
It's a vocation that has a requisite set of minimum skills that ideally start out with strong playing ability combined with excellent communication.
#1 Universal Rule of Tennis - The ball goes where the racket-face is pointed at contact. The player is responsible for working out the details like getting there, positioning, and creating enough force to redirect the energy of the incoming ball.
https://twitter.com/brentherrick/status/1394896793721262080?s=20
Coming up against blatant cheaters can take the fun out of tennis. It takes personal discipline to give your opponent the benefit of the doubt on line calls and I teach students to do so by telling them when in doubt call it in.
I think most know bad calls and cheating in tennis is a problem that is hard to solve. I've had adults and kids that I"m teaching make calls for their benefit that were obviously wrong. When I see it I talk to the whole group by having a quick line call clinic. The cheater knows I'm talking to them. In social tennis, you'll be ostracized eventually so it solves itself.
In competitive tennis, you need to take an active effort to stop it. If you have blatant cheaters probably the only way to fix them if you are the opponent is to point out your doubts on calls first, call an umpire, and if they persist, return the favor with obvious intent. A cheater can use such actions not only to take points but to break your focus so you need to find a way to fight them on it without putting yourself at a disadvantage.
The bottom line is that cheating does work for cheaters in competitive tennis so you can't just let it slide. You have to use every means available within reason to stop them.
1. question the calls first.
2. request a judge
3. quietly make it clear to your opponent that you won't be cheated out of a match and if that includes returning the favor then I'd liken it stopping a thief from stealing.
Maybe someday we'll have AI robots that can solve the problem by making calls for every court. Until then, letting a cheater win only makes you a loser.
You have to act or accept the loss.
Surprise, Surprise. Most of us know that too much or too little is bad for you in many ways. One trick is to stop drinking coffee or tea in the evening to help you get better quality sleep.