Tried but couldn't find where in the control panel or registry this setting is now.
just says "You might need to turn on Linking by UID setting on in the site setup if you are migrating from older Plone sites.".
https://github.com/plone/plone.outputfilters talks about "The captioning filter is enabled if there is at least one plone.outputfilters.filters.resolveuid_and_caption.IImageCaptioningEnabler utility whose available property returns True. This mechanism exists for compatibility with TinyMCE and kupu, which both provide their own control panel setting to enable the captioning feature.".