We should add comments or otherwise mark where Experimental XEPs are implemented in order to more easily find these places in the future. Experimental XEPs are subject to change and we might not always notice when they do or if they change in incompatible ways.
Therefore it would be good to make it easy spot these places so any changes can be surveyed.
I've marked places in mod_csi_simple, mod_carbons and mod_mam with comments where they rely on (other) Experimental XEPs.
Writing down the XEP version in doc/doap.xml also helps keeping track of what's implemented so that the proper versions can be compared in the future.