Looks very good, but I'm unsure if it doesn't just overlap with D6859, which also seems to include your edits?

It's okay if this patch is merged with D6859 (that's what it was originally meant to be), if you think a single large/combined patch is better.
Or we could split D6859 so that D6859 and this patch can be applied simultaneously without conflicts.

It would be nice to have a comment with some information as to why "6" specifically makes sense here.

Yes, it's mysterious. It's hard-coded 14 - 8.

This makes the margin between the icon and the text equal to the margin on the left, from the edge of the block and the icon.

UI_block_bounds_set_centered(block, 14 * U.dpi_fac)
style->columnspace = 8

Seems weird to have a split with a factor of zero, would a normal row work here?

uiLayoutSplit(layout, 0.0f, true)

The 0.0f means equal columns; also, this was used here until the last changes.

I tried Row a few times but it didn't work here for some reason.

/* Alert Icon. */
uiLayout *layout = uiLayoutRow(split_block, false);
uiLayoutSetAlignment(layout, UI_LAYOUT_ALIGN_LEFT);
uiDefButAlert(block, ALERT_ICON_WARNING, 0, 0, icon_size, icon_size);

Here "Row" and "UI_LAYOUT_ALIGN_LEFT" to avoid the icon stretching along the full width of the column.

It would be nice to have a comment with some information as to why "6" specifically makes sense here.

Although in this case it was calculated as described in the comment above,
but in fact it is just so that the text is not too close to the icon,
that is, I don't think it's necessary to strictly calculate the variable depending on others,
just some extra text padding from the icon.

@Harley Acheson (harley) Here's another one to commit if you have the chance. : )