Hello fest6,
On behalf of the team, thank you for your contribution and taking the time to improve Back In Time. We appreciate it.

Looks good on a first sight. Would you mind to place the setting in the "Options" tab instead of "Remove & Retention"?

Best,
Christian

This is nice and smooth. I like it. Just some minor suggestions.

Please also add a tool tip to the setting and its two widgets. Quick n dirty (with LLM help): Shows a warning when free space on the backup destination disk is less than the specified value.
If the Remove & Retention policy is enabled and old backups are removed based on available free space, this value cannot be lower than the value set in the policy.

I do have a helper function to handle long tooltips, wrap them and organize them in paragraphs:

# Two paragraphs
# Having two translatable strings is on purpose because it avoid escape
# characters (e.g. \n) for non-technical translators.
tooltip = [
    _('Shows a warning when free space on the backup destination disk is less '
    'than the specified value.'),
    _('If the Remove & Retention policy is enabled and old backups are removed '
    'based on available free space, this value cannot be lower than the value '
    'set in the policy.')
]
qttools.set_wrapped_tooltip(self.suWarnFreeSpace, tooltip)
qttools.set_wrapped_tooltip(self.cbWarnFreeSpace, tooltip)

You can try to add a changelog entry if you want. I'll rephrase it later if needed.

Do you plan any further steps within or after this PR?

We can merge it in this state and going further in a new PR.
The thing is that the warning currently occurres only when check-config command is used on the CLI.
I would like to have warning in the logs and notify system, when BIT runs via cron in background or without GUI in CLI. And additionally a warning dialog when starting (but before running) a backup job manually in the GUI. The GUI then should warn and ask the user to proceed with the backup or not.

But this can be realized in the next PR.