(See detail instruction shown below for more info, if you have time)
at this moment, i would suggest to use an alternative & trustworthy app that can connect with Google-Hangouts/GH/GT network, which is: Thunderbird app, it can connect with Google-Hangouts IM (instant-messaging) network (aka: web-service), or use your own choice of other app.Though its not a fully working solution, hopefully other users can collaborate & improve & find a complete solution. But user 'JBis' has shown at-least some partial way to copy specific files, settings, etc from older macOS to Mojave/Catalina.Google-Hangouts (aka: GH, aka: classic Hangouts, previously aka: Google-Talk, aka: GT) support was dropped from Mojave & Catalina, so 'Messages' app cannot use that web-service (aka: network) anymore normally, and a complete solution is not-found yet.
