Add Momoko to that list I believe. Can't remember when she got it done but it was this year.
I know wota and H!P don't give a crap about talent but it's in these girl's best HEALTH interests to get some better vocal coaches and teach non-damaging techniques. The fact that I even have to specify a coach needs to teach healthy techniques is bizarre but I've already seen clips of H!P's coaches advising things that damn near no one with a license would be advising.
I also think a huge part of what's causing this is girls being forced to sing too far out of their range - note that all of these girls are pretty decent singers, all of their techniques are not bad! Sayuki and Chisato especially do a lot of things really well when it comes to placing their voice, having a healthy vibrato, etc. But when they're going for those high belts, oh boy... I mean people acted surprised when Takahashi got nodules but I heard that coming for awhile because girl was sounding straight up strangled any time she was in her higher range (which was pretty much constant since she and Reina basically split those duties).
These girls are not being trained to mix their voice at all and that's a big part of the issue, imo. Mixing your voice is when you mix some head voice (also known as falsetto) into your chest voice. It thins it out and takes weight and strain off of your vocal cords when singing high notes. When you hear the big names belting (Mariah, Whitney, Christina, etc) - that's a mix. Sometimes a very chesty mix, but a mix none the less. Nothing will kill your voice faster than hitting all your high notes with a full chest voice. Even a natural soprano has no business hitting anything above a C5 without some mix going on but you will constantly hear H!P girls literally SCREAMING them. I have noticed Airi trying to employ a bit of mix in her voice for the last year or two but she is literally the only one I see purposely utilizing this extremely crucial technique.
Idols are not considered to be advanced singers, but idol music frequently requires advanced singing. It's usually very high pitched for moe factor, a lot of songs in this particular era of H!P have very little time for breathing, there are lots of rapid transitions between vocal registers. It's basically asking for vocal damage to throw these girls out there singing some fairly complex music without complex training.
I mean, it's tough - just because someone is trained to do something it doesn't mean they'll "get it". Even talented singers struggle to control the muscle and make it do what they want it to do. It's not like learning to do push ups. You're learning to manipulate a body part you can't even see from the outside. H!P probably sees it as a waste of money. They don't want to spend years paying for vocal training that shows incremental improvement when they only plan to get a few years out of a girl anyway.