Web3js – What Does Web3 Really Mean?

Straight from the docs here

Returns

Object|Boolean - A sync object as follows, when the node is currently syncing or false:

startingBlock: Number - The block number where the sync started. currentBlock: Number - The block number where at which block the node currently synced to already. highestBlock: Number - The estimated block number to sync to.

So it has nothing to do with the block from which geth will sync but rather the block at which the sync started. For instance, if you stopped syncing at 3500000 exclusive by turning your node off and then resumed, your startingBlock would be 3500000.

If MetaMask will stop injecting web3 this means web3.currentProvider is not gonna work anymore properly. My advice is to handle the change and not getting your app crashed by downloading your own web3 library and using ethereum provider. This variable was introduced in the latest versions. Example:

const Web3 = require('web3'); // import web3 v1.0 constructor from node_modules
if(window.ethereum) {
    const web3Instance = new Web3(window['ethereum']);

    var methods = {
        getAddressETHBalance: function(address)    {
            return new Promise(function(resolve, reject) {
                resolve(web3Instance.eth.getBalance(address));
            });
        }
    };

    methods.getAddressETHBalance(<ADDRESS>);
} else {
    alert('Missing MetaMask extention.');
}