I don't know if Blutooth is 4.0 or 5.0 but what I can tell is that the connection is much more stable on my sapphire F6 than it used to be on my 5S+. Whether it is with the phone or BT headset. The 5S+ used to lose the connection if I left my phone in another room. Now the connection remains even when I'm not in the same room as my phone (as long as I'm in, let's say, a 15 m range).
So yes, for me, the BT connection has improved a lot.

It certainly has improved but at a cost.
On my Fenix 6 Titanium:

Last night I was with BT on at 95% for 13 days projection.
So I have set BT off before to go to sleep?

This morning with BT off I got 92% of battery for 19 days projection.

BT can be a real battery drainer if the phone is in another room.

I believe early after release it was 4.2.  Definitely not 5.0.  I have been using wireless earbuds from Jaybird since 4.0 and normally the right earbuds are the control side.  So early on if you had your watch on left wrist and while running your left hand dropped behind your back you would get a dropout.  I was hesitant to buy the F6 due to them not including Bluetooth 5.0 (the specs and SDK were released 2016) but I did and honestly I am pleseantly surprised.  I have the Jaybird Vistas, Jabra Active 65t's, Jaybird Tarah Pros and Apple AirPods version 2.  Where the F6 on left wrist and do not really have any dropouts.  The Vistas have controls in the left and right buds so you can pull one out and still have music. I can force a dropout with the AirPods by placing my left wrist behind my back but other than that experience has been very good.  Minus the software updates which screwed things up for awhile.  But music and exercise is a battery killer.  Checked it one day I was at 99% battery to start.  At the end 2 hours later it dropped 15% to 84%.  Typically throw it on the charger every 4 days using GPS and music.

Thank you!

Thank you!

Thank you!

Bluetooth 5.0

http://www.f-blog.info/garmin-fenix-6x-pro-disassembly-or-teardown-whatever-you-say/

"Cypress CYW20719, BT 5.0 baseband processor with 2.4 GHz transceiver with BLE, EDR and BR. This seems to be the chip responsible for the Bluetooth music transfer to the wireless headphones and BLE connections. It has its own 96 MHz ARM Cortex M4 processor and has 1 MB flash, 512 KB SRAM and 2 MB ROM. It seems to be a capable chip, so it may even have more functions like playing the music tracks etc. Only Garmin knows the real functions…"

If your Garmin Fenix 6 X Pro ist from 2021 or 2022 it has Bluetooth 5.1

Where I know it from? See manufacturer Infineon, German company. There technical specification is seen at

www.infineon.com/.../

AIROC CYW20719 Bluetooth® and Bluetooth® LE System on Chip

• Complies with Bluetooth® core specification v5.1 with Bluetooth® LE 2 Mbps​

The CYW20719 is a Bluetooth® 5.1-compliant, stand-alone baseband processor with an integrated 2.4 GHz transceiver with Bluetooth® LE, EDR and BR.

Greetings from Bavaria, Germany.

Infineon Technologies CYW20719 BR/EDR/BLE Bluetooth 5.0 Drahtlose MCUs

https://www.mouser.de/new/infineon/cypress-cyw20719-soc/