How do I turn off sneak toggle into holding the sneak button?

Normally in Minecraft, you'd be doing this through the Accessibility menu - the little human icon right of the "Quit Game" button in the main menu; change 'Sneak' from 'Toggle' to 'Hold'.

...but this is a new feature, present as of 1.15. The only way to get equivalent functionality in an antique like 1.8.9 is through operating system's accessibility - you probably turned Sticky Keys on - pressing Shift five times and then clicking 'OK' instead of 'Cancel' on the prompt that shows up. To turn it off, open "Ease of Access Keyboard Settings" in Windows, then disable Sticky Keys - and the 'shortcut key' when you're at it.