I'd say about 50/50... not sure anymore. The hidden serves are the ones that were played 15 years ago, with the arm in front to hide the contact point. I only played him once so far and probably won't for some time as his clubs is pretty far away.
The big guy on my club whose looping game people like, he habitually begins his serving motion holding the ball beneath the table.
There is another guy from India I play whose major training period was during the hidden serve era. So even if he is trying to serve legally, his pendulum serves are hidden and his backhand serve is low toss. He went to university in India on a TT scholarship so good luck training those serves out of him.
There is another guy with relatively low toss backhand serves. He has only beaten me once in all the times I have played him but he loops and smashes on both sides and he is a tough customer if you let him get his shots in.
There is a guy on my usual Tuesday league with an amazing forehand. He doesn't know how to toss the ball high enough to serve legally. Fortunately he doesn't know how to loop heavy topspin so I beat him by putting slow heavy topspin on the ball and keeping it short who his powerful forehand puts the ball long repeatedly.
There is a lefty who is a buddy of mine. He doesn't deliberately hide his serve but he often serves and forgets to remove the arm. He complains about other people's serves as well so you get an idea of what I mean when I say that many people have never seriously scrutinized their own serves and few people serve legally. This weekend he received a few complaints and removed the arm and people still had trouble with his serves (they are heavy and lefty and he has a mean no spin serve as well to go with the heavy backspin).
There are probably a few more stories I could tell. I have beaten all these players despite their illegal serves.
I even had a match at Joola teams where one player who was also an ump (certified) was leaving his free arm in the way. I lost that one partly because I couldn't make head or tail of what was going on. But I have resolved to watch the flight of the ball more. When you watch people play or when you play, try to imagine how you think the ball is spinning or call the spin out in your head before returning the serve so your response can be grooved into an automatic stroke. Usually backspin or no backspin is often enough. The main thing is that you can't block no spin.
Many people who played before the year 2000 don't complain about illegal serves that much. Even if they were only beginners. They got ball flight training. They are not happy with enforcement but they just play through their matches and fight to read the ball and take the ball late.
Boogar, I recommend you listen to DerEchte and take all illegal servers as people who are bringing you a gift. Trust me, I am a terrible serve returner relative to my level and I have actually played matches where my knees were better and I would just lob the serve and step back and play goal keeper. You will be surprised what you learn about yourself if you give yourself a chance to try stuff.
Because the 2200 guy is hardly the problem. It is the 1800 guy who is serving illegally and you can't pick up what to do. Then you remember he is 1800 so he can't be so good at it that he beats everyone. That 2200 guy have you a gift of realizing that you are not so bad that you cannot return any serve just because you can't see it. So you focus and look for clues. And if it is a best of 5, your brain will find clues.
I was playinf this very aggressive 1700 guy in a league match. I was the top player and I was above everyone by 300 pts so it was supposed to be an easy day in the Saturday league. Then I am struggling to return this guys serve. And when I pop it up he smashes or loops or so hard I have no clue what to do. I honestly do not know how I played that match. The biggest thing was that I needed to loop the serve but I kept telling myself he was 1700 and he had to miss. By the time I had lost the first game and was down in the second game I realized I had to loop the serve even if I lost. By some miracle I won game 2 11-9. And I started seeing the serve better on game 3 and 4 (Don't ask me how or why - I tell people that your brain just figures it out but you will block it from doing so it you continue to complain as you won't realize it and will play with frustration) and I won the last two games comfortably. I found one serve he didn't like and that helped me too.
By the way, all these matches are on my channel and so are all these players in case you think I am being too dramatic. Even today. I lost to someone who remembered my old serve return issue with the forehand which I have improved a lot and he exposed a new one in part because my shoulder is injured - no spin serve to the middle. Back to the videotape.