Mental health professionals use died by suicide. This is something you can verify with a simple google search.

Google results -- this seems to confirm that it's jargon and not standard English.

And we're not having this discussion in a clinical context. The unusual usage of "completed" was on a Twitter post, the discussion of the usage was here in a Reddit thread. When people pointed out that it's a weird usage, others responded by describing what they think is "the preferred language now" as a general statement without any relation to the narrow context in which that may be true.

The r slur used to be standard vernacular for mentally challenged people, but now it is considered outdated and offensive.

This is a little closer to the truth, as there's definitely a marked downward trend of the "'r' word", although it's still the most common term in general use.

Language is constantly changing and evolving along with society.

Absolutely, but that's a real phenomenon that's observable and measurable, which gives us insight into what the language actually is at any given moment. Saying "language evolves" isn't a meaningful excuse for using language in a way that's inconsistent with what actually is the current state of actual usage, or pretending that novel usages that are limited to specific niches are somehow "correct" while what remain the most widespread and standard usages are somehow "Wrong".