Obsidian Sync will keep the version history up to a year. Encrypting a small part of content in the note cannot erase it from the revision history.
Proposed solution:
Allow a prompt to let the user input the content and directly insert the encrypted text into the note. This way, the version history will not keep the plaintext.
from v2.3.6 if you invoke the Encrypt/Decrypt command with no text selected, you will get a dialog with a section to enter the text to encrypt directly into the note
Context:
Proposed solution: