I updated to Plasma 6.7 and it seems I’ve “unlocked” the feature where holding a key gives a popup to select accents. As someone who speaks English only though, I find this annoying and it is intrusive when I hold a key for a little too long. My problem is, I can’t find the setting to disable this! Where is the setting so I can get rid of this feature?
You could disable Plasma Keyboard entirely. (In System Settings, search for “Virtual Keyboard” and change the setting from ‘Plasma Keyboard’ to ‘None’.)
As far as I can see, you can’t disable just that feature while keeping Plasma Keyboard enabled.
That fixed it! I didn’t know Plasma Keyboard was what was causing that - there’s no indication it would. Thank you!
You can actually : In Keyboard > On-screen Keyboard section, you have the “Alternate characters - Show popup when holding a key” toggle.
Ha! I thought I tried that but I must have failed to click “Apply”! 
Thank you. I got bitten by this - in logseq my dead keys no longer functioned correctly. Then in another app I suddenly got the accented characters popup which lead me here.
Honestly, that setting should only apply to the on screen keyboard by design. It doesn’t. So when I press keys on my physical keyboard I get the popups, which is very annoying…
It’s a new feature for physical keyboards, and it is intended, mostly targetted at people with dense diacritic languages and having trouble memorising the layouts. It’s an accessibility feature after all. Plasma Keyboard goes farther than just an on-screen keyboard by trying to provide a fully fledged IME experience.
Otherwise, the toggle only toggles the physical keyboard feature, which… yeah it’s on the “On-screen keyboard” page and the wording seems to confuse a lot of people.