Look at Woodcutter. If you have a tile with a Woodcutter on it, the mouseover text for the tile says
Must research Machinery to upgrade (see screenshot).
View attachment 404844
However, Lumbermill does not have an <ImprovementPillage>. Therefore, the Civilopedia tech description for
Machinery does not include the text
Allows Woodcutter to upgrade to Lumbermill. The
Allows X to upgrade to Y text only appears if Y has an <ImprovementPillage> to X. If improvements Z or W also upgrade to Y, they will not appear in the Civilopedia tech popup.
This is how the mouseover popup for Machinery currently appears:
View attachment 404846
If I give Lumbermill an <ImprovementPillage> to Woodcutter, it appears like this:
View attachment 404845
I want every tech that allows an improvement upgrade to show all improvement upgrades like the last screenshot, but for every upgrade possible. It's not currently reflected now, but as an example, Lumbermill, Orchard, and Forest Preserve are all going to be able to upgrade to Hybrid Forest and I would like to be able to see that in-game. I don't think we need <ImprovementPillage> for all improvements.