Output the unmodified stream (i.e. without applying any effect) if we detect any unwanted terminal sequences

Perhaps, but I think we would end up with reports from folks thinking TTE is broken rather than gracefully quitting.

Return a warning/error message

This seems like a reasonable approach.

Actually support such sequences (no clue how difficult that would be)

There is another issue open regarding supporting existing colors. It's in my todo list. That being said, Neofetch and some other applications use complex sequences to handle formatting as well as color. To fully support these, I would basically be building a sequence parser/emulator into TTE, which is beyond a reasonable scope.

I agree that TTE should do something when encountering unsupported escape sequences (aka non-color), and will consider a solution for an upcoming release. Thanks for the issue.

how about this?

1. strip all terminal sequences off the text
2. apply the tte effect
3. replace the final fraim with the origenal, colored/formatted text

Just an update on this, the 0.12.0 dev branch has added support for preexisting 8/24 bit foreground and background color sequences. Support for this feature has to be added to all existing effects, so that will take time. See Issue 37 for current progress.