To me, Monorepos are about a separation of concerns (or lack thereof). Something is a monorepo if/only if there is greater than one concern in the repo. (For example mobile + web, backend + frontend, service + server).

When I was at FB, I remember someone used the term polylith to describe our multiple monorepo situation... I suspect that's probably a pretty common situation for people that "have a monorepo"

The number of secondary repos was/is declining by merging them into poly-monorepos. For example, opsfiles was/is moving to fbcode.

With good D/VCS tools that can checkout code efficiently, review processes, and CICD testing, there's no real reason to avoid monorepos.

Btw, the FB "hg" was semi-opensourced as sapling. Eden and mononoke still haven't been FOSSed. And, like many Meta dev tool FOSS projects, they don't just build and work OOTB and often have hard-coded Meta-isms like references to internal paths. https://github.com/facebook/sapling