Well, it does look like we last updated unimod.xml in November 2017 but to a version that contains changes only up to 3/10/2017. And it looks like Unimod ID #35 (Oxidation) was most recently updated on 10/6/2017 to add a number of new amino acid residues including S:
<umod:mod title="Oxidation" full_name="Oxidation or Hydroxylation"
username_of_poster="unimod"
group_of_poster="admin"
date_time_posted="2002-08-19 19:17:11"
date_time_modified="2017-10-06 17:05:11"
approved="0"
record_id="35">
<umod:specificity hidden="1" site="T" position="Anywhere" classification="Chemical derivative"
spec_group="18"/>
<umod:specificity hidden="1" site="E" position="Anywhere" classification="Chemical derivative"
spec_group="13">
<umod:misc_notes>hydroxyglutamic acid</umod:misc_notes>
</umod:specificity>
<umod:specificity hidden="1" site="S" position="Anywhere" classification="Chemical derivative"
spec_group="17"/>
<umod:specificity hidden="1" site="Q" position="Anywhere" classification="Chemical derivative"
spec_group="16"/>
<umod:specificity hidden="1" site="L" position="Anywhere" classification="Chemical derivative"
spec_group="15"/>
<umod:specificity hidden="1" site="I" position="Anywhere" classification="Chemical derivative"
spec_group="14"/>
<umod:specificity hidden="1" site="U" position="Anywhere" classification="Multiple"
spec_group="12"/>
<umod:specificity hidden="1" site="G" position="Any C-term" classification="Pre-translational"
spec_group="11">
<umod:misc_notes>Hydroxyglycine derivative in amidation pathway</umod:misc_notes>
</umod:specificity>
We will look into updating our unimod.xml file, but for now, you could also remove the offending rows from the CSV file that include S(UniMod:35). Hopefully, S oxidation is not an important target for you.
Sorry for falling behind this library. Thanks for letting us know it is time to do another update.
--Brendan