Guided lactate threshold test - Is it gone???

Same here.

I suppose it has been replaced by continuous recalculation of HR zone after activities

Based on my email with Garmin support and my own experiences, the new continuously updated LTHR method is much more complicated than the old one, which just gave you a new value based on the test run. It seems to continuously analyse different metrics and adjust the LTHR based on changes in those. I haven't figured to sure way to trigger a new LTHR detection, but that's probably because as I just wrote, it's not any more something the watch gets from a single run only.

But a funny thing I've noticed myself is that if the watch "confirms" the current LTHR (i.e. something triggers the detection but the result is the same as before), the watch doesn't notify about that and the date on the Performance glance doesn't change, but you'll get a new data point in Garmin Connect. I guess this is intentional, since the watch then tells me only when the LTHR has changed.

Why the old LTHR test was removed is of course something only Garmin could tell, but my guess (as a software person) is that the new continously updating LTHR analysis and the old one-run-to-rule-them-all were not compatible with each other.

Ok, but that's not so good :-)

My lthr and pace is old and way too optimistic, so I just wanted do a guided test, to get back to reality. The way I work out at the moment with intervals indoor and zone 2 outdoor, doesn't allow for Garmin to change my LTHR/PACE, because now I have to do lots of outdoor sessions where i get to or over lthr i guess, because it doesn't do it from one run, but many runs over time???

To me it seems like the current LTHR estimate tries to estimate both LTHR and LT pace also from slower activities, just like the maxHR detection doesn't require you to reach your absolute maximum HR (which is often impossible in practise, anyway).

But yes, it's far from perfect. Now in the summer it's been giving quire realistic estimates for me, but during the winter when I ran in sub-zero (Celsius) temperatures, it sometimes gave quite inaccurate values, both the HR and pace (the latter I can understand because I was running in snow, but it's stil irritating because there seems to be no way to turn the detection off completely).

To me it seems like the current LTHR estimate tries to estimate both LTHR and LT pace also from slower activities

If that was true, it would have down-regulated mu lthr and pace....at least I would think so :-)

If you turn off performance metric calcs for Trail Run type activity, you can select 'Trail Run' for winter runs in the snow or when bundled up with tons of clothes etc.  Seems to not calc LTHR from what i can tell with my recent training with plenty of Trail runs building up for a trail ultra.  I also log Trail Run when running road with my hydration vest (plenty of extra weight).

Unfortunately that's what I tried to do. I turned off VO2max calculation from trail running, but still got an update on LTHR and LT pace from a winter trail run (and the pace was of course way off). I contacted Garmin and got a confirmation that LT updates are possible even with VO2max detection off in the activity settings. So you'll actually have to turn off the global LT auto detection setting to prevent LT updates, there's no per activity setting for that.