No, you remove or add spacers to the shaft of the air spring so that it can extend/compress less/more. That reduces/increases the travel. You do this with the fork empty of all air, because you have to physically remove the air spring shaft from the upper chamber to do it. You then put it all back together and pump it up to the same sort of pressures you had before you removed or added the spacer.
Running a fork with the same length air spring sagged by removing air doesn’t affect travel , as the fork can still move between the same points. It will ride lower due to the sag, but the total available travel from unweighted to fully compressed isn’t affected.
Does that answer your question?