With the WWDC and the announcements of the successors to MacOS 10.14 (Mojave) and iOS 12 barely a month away, it is time to get serious about getting ready for an all 64 bit world. It isn't just applications that are affected by Apple dropping 32 bit support in MacOS 10.15, the successor to Mojave. Some of those incompatibilities specifically impact the Audio, Video, Photography world.

Apple has has announced in Support Document HT209594 that Aperture will not run past Mojave. The support document includes instruction on migrating the Aperture Libraries to Photos or Adobe Lightroom Classic.

Some older iMovie Libraries and Final Cut Pro and Motion Libraries will also have to be converted. Conversion instructions are found in the linked Apple Support documents.

NOTE: as far as I can tell the 32 bit libraries and projects are not listed by either System Information or Etrecheck.

Is it reasonable to think in MacOS 10.15 that in the case of Apple libraries (e.g., iMovie), when the user tries to access them they will be given a prompt to convert?

If it were me, I would follow Apple's instructions to convert them now. It could be 10.15 will not be able to read the files to make the conversion, but I don't know that for a fact.