Are you lost? Discord is in the Arch community repo as already mentioned. It's an Electron app, so installing it ahead of a Manjaro stable update will do no harm. However, with a lot of other applications there could definitely be an issue doing so. In that regard, you have a very valid point.
FYI, I maintain over 100 (I lost count) AUR packages with mostly no issues using Manjaro testing.
...and we're getting too much into a discussion in a thread where that's considered off-topic.