There here is, a rewrite of my unrelease script : SeatbeltR (Video Preview)
The quality of SeatbeltR is suppose to be paid but I put it on hold because of it’s ugly code.
In these day, I rewrote it and decided to release it for free.
The weight part and the damage part in video is removed.
Github: SeatbeltRW

Free

It is free,You can still buy some Coffee/Bread for my job on the Tebex

Installation

server.cfg

setr game_enableFlyThroughWindscreen 1

start SeatbeltRW

Get Player Seatbelt State

You can use GetPlayerSeatBelted or GetPlayerSeatBeltedByPed to get if the player is seat-belt equipped.
by

Method 1

Import :

load(LoadResourceFile("SeatbeltRW", 'import'))()

Example :

load(LoadResourceFile("SeatbeltRW", 'import'))()
CreateThread(function()
        
        while true do Wait(0)
            print(GetPlayerSeatBelted(PlayerId())) -- false or true
            print(GetPlayerSeatBeltedByPed(PlayerPedId()))  -- false or true
        end 
 end )

Method 2

exports['SeatbeltRW']:GetPlayerSeatBelted(playerid)
exports['SeatbeltRW']:GetPlayerSeatBeltedByPed(ped) 

Make sure load SeatbeltRW before you Get State or Import.