I don't think anyone anywhere has ever said the phrase, "Let me text you that calendar file." Perhaps it was commonplace practice before the era of Google Calendar and other cloud-synced tools, but it's hard to see a case for texting calendars in 2018. Still, someone at Google must have thought that functionality would be useful, because it just arrived in Android Messages.
The latest update for Messages, version 3.9.039, includes "You can now share calendar file formats" in the changelog. You can't actually do this from inside the application, though — you have to share the desired calendar from a file manager, and Messages will appear as an option. I tried downloading a calendar of U.S. holidays from iCalShare (anyone remember that site?) and sharing it from the OnePlus File Manager, which seemed to work fine.
I know you're all excited to try this out, so feel free to grab the latest version from APKMirror.