RESTBase is a store, not a cache. As such, there was on reliable way of knowing when old HTML versions would clear out from it and we also didn't have a mechanism to purge old versions. As such, Parsoid's codebase has a number of FIXMEs / TODOs to get rid of some code when the HTML for a version turns over in RESTBase.

But, we are now moving away from RESTBase to ParserCache where all entries turn over after X days (currently X = 21). So, once we fully switch over all wkies to ParserCache, after that X-day period, we should be able to get rid of all the b/c code in Parsoid that exists to deal with older HTML versions.