There are escape codes that can re-define palette entries. Usually including the 16-255 range.

Yes, I know that (see the venerable https://github.com/trapd00r/colorcoke, etc.) but those tricks are not used widely enough for them to be a concern. Using those tricks is also a deliberate choice so it is definitely on the user if my lovingly crafted 256c colorscheme is broken.

Having all terminal emulators run the equivalent of colorcoke without asking the user is not a very bright idea.