If you look at the history of certain pages which were originally imported from Wikispaces, you will see an revision in the listing from the user "Wikispaces>FREEZE." This signifies that the revision in question was current when the Wikispaces was frozen for editing (July 16, 2018).
Some people have used the term "FREEZE" to refer to the parser converting from
wikispaces to MediaWiki, although this was a loose informal effort by Mike and Tyler and not a finished "engine" of any type.
Any formatting created by the "FREEZE" process was a heuristic (i.e., the best thing we could do at the time) and was not meant as a standard.