Château can be built on Floodplains without being adjacent to a bonus/luxury resource

[Infixo]

This is from CivFanatics: https://forums.civfanatics.com/threads/ch%C3%A2teau-can-be-built-on-floodplains-without-being-adjacent-to-a-bonus-luxury-resource.683437/

[Infixo]

-- for future reference
In GS (1.0.0.290) they changed plunder amount from 25 to 15 and made it possible to build Chateau on floodplains
In April 2019 (1.0.0.314) they changed plunder type from culture to faith, added yield adjacency for wonders and luxuries
In June 2019 split wonders into early and late
In April 2021 (1.0.12.9) they removed "requires river" and added "same adjacent valid = false" and "requires bonus or lux = true", and XP2 yield adjacency were moved to the base game

tl;dr In the base game you can't build it on floodplains, in GS you can. However, in the base game you needed river, now floodplains are enough.

The issue is imho with the Improvement_ValidFeatures table. It has higher priority than reqs placed in the main Improvements table. Ofc, we don't know if by design or by mistake. So, since GS allows to build Chateau on Floodplains, then it is enough. Lux/bon adjacent is not checked.
Reverting this back is not reasonable imho, since Features block placing improvements, so one couldn't place a Chateau near the river in many cases, which is like their most known characteristics. So, floodplains need to stay...

All in all I think this is by design. In the base game you needed a river to build it, and floodplains were rather rare (I think?) In XP2 floodplains are quite common, meaning there is a river. Imho, "requires river" was traded for "floodplains".
To balance (but same problem was in the base game - you could place them one by another along the river) they introduced more req - no adjacent chateau and needs a resource. Plus some bonus from such situation.
Ofc this means that you can also build it far away from the river... but I suppose there are Chateaus in France not by the river?

The only change that could be done is re-introduce "river adjacent" to forbid away-from-river placement. I'd rather just clarify the description by mentioning than you can always build on floodplains.