Is there a developer mode or developer menu for Garmin Forerunner 965?

Can you post the procedure here so that we can try it too? thanks

I have tried instructions in this video https://www.youtube.com/watch?v=rGiPCtllpIQ on Youtube, it does not bring me anywhere other than a support page with"clear Ephemeris" and "Reinstall Software".

What are clear Ephemeris?

Not on the newer Garmin wearables. Fenix 6/6X have a development menu. Below is a list taken from my F6Pro years ago, combined with options I found on my F6X today (much never firmware), some options were not on the F6X. I added what I have found on information. Maybe you can get a used F6 for a nice price if you need access to any of the things listed here.

Get into the ABOUT menu, Settings - > About or Settings -> System -> About
Press the light button eight times

Options on Fenix 6 Pro:
- Force Crash: Reboots the device, after writing RAM.txt and err_log.txt in Debug.
- Save RAM region: writes RAM.txt and err_log.txt in Debug. err_log.txt contained stale error info from last spontaneous crash.
- Dump GDB: writes gdb_dump.csv in Debug. This is a listing of all fit files (sports profiles, activities, workouts, settings etc) on the device, per directory.
(Only F6X): Dump OTD Snapshot
- Dump Power Modes: writes power_modes_dump.csv in Debug. Overview of how power modes are configured.
- Enable/Disable BT Logging: Bluetooth logging
- Enable/Disable ANT Logging: ANT+ logging. Writes ANTPKT1.fit in Debug, viewable as csv after using fitcsv tool from FIT SDK
- Enable/Disable PTS Test Mode: (Probably) Bluetooth Profile Tuning Suite, a testing tool provided by Bluetooth SIG
- Playback: select a log to playback
- Record Log: "Bin Log Setup": options are: WDR, SNS, Firstbeat, FTM, GPS. This starts an activity, the last used sports profile is used.
(Not on F6X) - Alt. and Weather: Allows manual setting of altitude, heat and humidity, units unknown
(Not on F6X) - Stress Testing Override Off/On: This sets one of these statuses in the Stress Widget. Options are: Too active, Recovering, Not Enough Data, Unrecognized
- Clear Cal Points: Unsure. Options are Yes and No
- Easyhunt Server PROD/TEST: Used for ETRACK+ dog tracker
-> F6X: Translation Files Test
- Demo Mode Off/On: To simulate having a valid GPS position fix, in order to evaluate or practice with a device indoors.

Another nice feature was the Service menu. It was removed from Fenix 6-generation in later firmwares. It is now only available on Fenix 5-generation and earlier:
turn OFF watch (press light button and select power off)
HOLD down the DOWN button, (bottom left) don't let go
while keep holding DOWN, power on by pressing light button and let light button go
wait a bit, don't let go of down, eventually service menu comes up

The Service menu is for testing display, buttons, sensors, battery. It can show XYZ accelerometer values for example.

Regrettably all that fun is gone from current wearables.

Hi Bit, thank you very much for your answer! I am currently working on a project that need acess for accelerometer and gyroscope data, and that the reason I was looking for the developers mode. I am programming an app to recorde those raw sensor data but I am not able to access those sensors. By any chance you know this area? 

As far as I know there is no direct access to accelerometer data from within the ConnectIQ environment. You could try and ask on the developer forum.

There is another brand watch but that is probably not what you are looking for, the Texas Instruments eZ430. That has direct access to the accelerometer data.