A journal appeared indexed until 2016 is now cancelled in Scimago. What does this mean?
Scimago seems to draw from Scopus' journal coverage (according to its About Us-page):

The SCImago Journal &amp; Country Rank is a publicly available portal that includes the journals and country scientific indicators developed from the information contained in the Scopus® database (Elsevier B.V.).

Scopus regularly re-evaluates its journals. It even offers an Excel-list of recently discontinued journals (at the bottom of this page).
In that list, you can see the column Reason for discontinuation (column F). This should at least provide a vague keyword as to why a given journal was cancelled.
It seems that there are four such reasons:

&quot;Metrics&quot; (under-performance of a journal)
&quot;Publication concerns&quot; (I see journals from predatory publishers like OMICS who carry that reason).
&quot;Radar&quot; (outlier performance; &quot;an algorithm that flags journals based on approximately 40 outlier predictors, including sudden change in output volume, sudden change in publishing country and/or affiliations, and high journal/author self-citation rates&quot; -- citation from this F1000 article, at p. 14)
&quot;do not meet Scopus criteria&quot; (probably refers to Scopus' journal selection criteria, e.g. peer reviews, lack of publication delays, editorial board diversity, online availability of articles etc. As this Scopus blog post indicates, some journals are indeed simply discontinued by the publisher, which might fall under this reason.)