1. Open [System Preferences]-click [Software Update].
2. Deselect [Automatically keep my Mac up to date].
3. Then click the [Advanced] button to cancel all the checks.
4. After setting through the above steps, if you find that the small red dot at the bottom is still there, you need to open the terminal.
5. Open the terminal and execute the following code.
Ignore the major version update prompt:
sudo softwareupdate --ignore "macOS Catalina"
How to ignore minor version updates:
sudo softwareupdate --ignore "macOS Catalina 10.15.4 Update"
6. Cancel the little red dot
defaults write com.apple.systempreferences AttentionPrefBundleIDs 0
killall Dock
7. Resuming update prompt
sudo softwareupdate --reset-ignored
defaults write com.apple.systempreferences AttentionPrefBundleIDs 0