It was always junk. 'Cleaning' the registry, which has always been snake oil at best and potentially break Windows at worst. Even Microsoft doesn't recommend registry cleaners.
The only disk cleaning tools you need are provided in Windows by default.
10+ years ago it was okay. Today, not so much. And while the registry "cleaning" might never have been "useful", I did use it on a "why not" basis and didn't have an issue, ever. Also noticed no positive effects.
But either way, the Microsoft that makes random hardware demands for Windows 11 to turn computers into landfill isn't what I would take recommendations from. Or much anyone else, either. Just by using Revo Uninstaller I am just constantly amazed at how much stuff software leaves around in the registry and on disk. I recently uninstalled Keybase because I wasn't really using it but noticed it had accumulated 200MB of log files (for nothing, for running in the background basically), and after uninstalling it, RU found an additional 400+ MB of leftover files. And that just rubs me the wrong way, and even if it doesn't measurably slow down the use of the system, just having all sorts of orphaned and 100% useless data checked for changes every time a backup runs is reason enough for me to get rid of it.
FWIW, my system doesn't seem to get slower over time, and I do attribute that to caring too much about and fiddling with things I'm not supposed to care about. I don't recommend it to others -- if you know enough and care enough to do it, you're doing it already -- but you can pry that from my cold, dead hands.
Systems don't really get slower over time because of used disk space. It used to be somewhat true when HDDs, and bad ones at that, were more common. Nowadays though what's left on your 2 TB <$70 2 GB/s SSD storage matters little so long as your drive isn't near completely out of space to the point it can't place things easily. Background apps/injections are another thing though. You can never uninstall an app for 10 years and your computer will be none the slower for it as long as you disable them from automatically starting up, loading 5 background services all the time, and having poorly written hooks into things like Explorer which constantly get loaded every time you open a file dialog.
Tinkering for tinkering's sake can definitely be fun though and there's nothing wrong with that. Sometimes poking at things, and sometimes breaking them, is a great way to learn new things or even just scratch a casual itch.
Yeah I kinda conflated two things there. One thing is that do I do poke at things a lot, I regularly run Sysinternals autoruns just out of curiosity -- which also "can potentially break windows" (what can't?), poke around in Process Explorer or even run Process Monitor occasionally to just filter out everything for which I know the reason, to see what else is going on. I remove stuff from context menus I don't want there, you name it. All those little things do add up, even if it just reduces the number of things I have to scroll past when doing other things. But I admit that I mostly enjoy doing it.
But while SSD changed everything obviously, in addition to daily partition-based backups I also make manual backups to my NAS with FreeFileSync, just so I don't have to restore full disks when I only want to pull a few files from the backup. While FFS is crazy fast checking for changed files, all those orphaned files are still not free... it's not about the size so much as about the number of files, doubly so when it goes over the network. I also rather often search my whole system for stuff, and there too, files and directories that don't exist are the quickest to search.
I wouldn't want to keep them around in the same way I wouldn't want to run a script that puts 100k tiny files with random content all over the place. Because it's just a pointless waste. And since I enjoy the process, I don't count the time I am using on that, so in my mind, I'm being frugal in a weird, misguided sort of way... I know I could spend the time to make more money and then just buy another HD, but I don't wanna :P
Especially when you make your own cleaners [0][1], which you can also can selectively invoke via the command line (and of course do that via schedule / cron etc.)
The only disk cleaning tools you need are provided in Windows by default.