Wikisource and Wikibooks do not use OCG for most of their PDF rendering. This pause of functionality will only impact PDFs created through Book Creator, and not impact the workflows you describe as you're using different rendering services for PDFs. Books previously generated using OCG will not be impacted, either.
Wikibooks does have the book creator tool, called "Creation Collector," which will be impacted. I'm not sure how much it's used there.
Unfortunately, OCG is in a state where it can't be maintained and we need to deprecate it in order to maintain security. There are other reasons for moving to a new service. For example, OCG can't render tables. If you look at the mediawiki page I linked earlier[1] you can see the rest of the rationale and the timeline for the changes.
I'm adding Olga Vasileva, the product manager leading this work, for any further questions.