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A modern paper
(fedia.io)
It feels ridiculous to see a 5-page paper with a 150-page appendix. Makes you question what is the paper.
As it should be. There's no point doing research if you don't publish all the relevant information. Now that journals are electronic, you can and there's no excuse not to.
If you don't know why the appendix exists, try reading it.
No need to rush to conclusions, I do read appendices when needed. My point is not that authors should cut the appendices or compromise on any other good open science practices. The point is that disproportionately large appendices make one wonder if some of that stuff actually belongs in the main text. If it is just a robustness check that gives a similar result, fine, make a footnote in the main text and put the analysis in the appendix. But what if it is actually relevant information that changes the perception of the main text?
That's why you need the appendices, so that you can check the details behind what is in the paper.
Journals have word limits, due to the restrictions of print, and because a 200 page paper is too much for most readers. But some of them will need some or all of those 200 pages (which is usually a shed load of tables and figures, not much text apart from protocols etc).
The quality of the research, and the way it was written up, cannot be assessed by those readers unless all the information is published. And the research cannot be implemented in practice unless it is described in full. There are thousands of papers out there that test a new treatment but don't give enough detail about the treatment for anyone else to deliver it. Or develop a new measurement scale but don't publish the scale. Or use a psychometric instrument but don't publish the instrument. This research is largely useless (especially if the details were never archived properly and there's no one still about who knows how to fill the gaps).
We don't (or should not) publish papers for CV points. We publish them so that other researchers know what research has been done and how to build on it. These days we don't just publish all the summary tables and all the analyses, we ideally make the data available too. Not because we expect every reader to want to reanalyse it but because we know some of them will need to.