I can pinpoint when it stopped being great, which was after the 7th season; although season 8’s “Homer vs. the Eighteenth Amendment” was a masterpiece. When I quit I don’t remember exactly, but among the last few episodes I recall Homer smoked pot and the family went to Brazil, I think.
It wasn’t the liberalism, rather the endless self-reference, lack of plot, and increasing unfunniness (though this is subjective) that turned me off.
To alcohol, the cause of and answer to all of life's problems.