"As part of inking and typing on your device, Windows collects unique words—like names you write—in a personal dictionary stored locally on your device, which helps you type and ink more accurately.
If you sync your Windows device settings to other Windows devices, your local user dictionary (up to 100 KB per language and 300 KB total of hard drive space) will be stored on your personal OneDrive for the purpose of enabling sharing of your dictionary with your other Windows devices. "
And it may have been worse when Windows 10 was first released:
"As part of inking and typing on your device, Windows collects unique words—like names you write—in a personal dictionary stored locally on your device, which helps you type and ink more accurately.
If you sync your Windows device settings to other Windows devices, your local user dictionary (up to 100 KB per language and 300 KB total of hard drive space) will be stored on your personal OneDrive for the purpose of enabling sharing of your dictionary with your other Windows devices. "
And it may have been worse when Windows 10 was first released:
https://www.pcworld.com/article/2974057/how-to-turn-off-wind...