21.19 seems to be released

See the post from 13 hours ago: forums.garmin.com/.../forerunner-255---software-version-21-19---20-phased-rollout

This post was written before Garmin published the post you mention.

Is the Guided LTHR Test present under run activity settings?

No, I can no longer see the LTHR test.

Can someone explain in detail what the following improvements consist in?

• Adds compatibility for Garmin Coach strength training plans
• Improvements to support Pool Swim workout with pace alerts and critical swim speed
• Improvements to the Pool Swim rest screen and alert tones

Yup its gone.  Shame as it produced the most accurate results.
The "auto" thing ive found useless.  The LTHR varies by 7 or 8 bpm depending on what activity ive been doing at the time, how hilly it was, what surface i was on and various other things.

At least the guided test produces a more plausible, accurate result on a controlled run on a controlled course/road.

So far last week i had LTHRs ranging from 172 to 165 in the space of 4 days with auto measure due to different terrain and surfaces.

Maybe best to run a controlled test run, then disable auto updates of LTHR until the next controlled run.

totally agree. quite a lot of Garmin users in China SNS complain that new firmware show them an impossible LTHR, much higher than usually with Garmin chest belt.