This is a NodeJS script to clean up our remote git repository by deleting branches we longer need. I have commented out the line that does the actual deletion for the time being.

cleanup-git-repo.js

#!/usr/bin/env node

'use strict';

const fs = require('fs');
const readline = require('readline');
const execSync = require('child_process').execSync;

const config = {
	filterText: 'DISCO-',
	mergedBranches: `${process.env['HOME']}/merged_branches.txt`,
	notMergedBranches: `${process.env['HOME']}/not_merged_branches.txt`,
	notMergedOldBranches: `${process.env['HOME']}/not_merged_two_month_old_branches.txt`,
	branchAge: '2 month ago'
};

/**
 * @param  {file} - path to file that contains lines of branch
 * @return {string[]} An array of branches
 */
function fileToArray(file) {
	try {
		let branches = fs.readFileSync(file,  'utf-8').trim().split('\n');
		return branches.map(branchName => branchName.trim());
	} catch(err) {
		throw err;
	}
}

/**
 * @param  {branchName}
 * @return {Boolean} - if branch is older than config.branchAge return true
 */
function branchIsOld(branchName) {
	try {
		let commitsBuffer = execSync(`git log -1 --since='${config.branchAge}' ${branchName}`);
		let commitMessages = commitsBuffer ? commitsBuffer.toString().trim() : '';
		
		// if we don't find any commit that is 2 month or less old, then it is old
		if(commitMessages.length === 0) {
			return true;
		}
		return false;
	} catch(err) {
		throw err;
	}
}

/**
 * @return {Array} - Array of branches that are NOT merged to develop
 */
function findNotMergedBranches() {
	try {
		execSync(`git branch -r --no-merged develop | grep ${config.filterText} > ${config.notMergedBranches}`, {stdio: ['ignore', 'ignore', 'ignore']});
		return fileToArray(config.notMergedBranches);
	} catch(err) {
		throw err;
	}
}

/**
 * @return {Array} Array of branches that are merged to develop
 */
function findMergedBranches() {
	try {
		execSync(`git branch -r --merged develop | grep ${config.filterText} > ${config.mergedBranches}`, {stdio: ['ignore', 'ignore', 'ignore']});
		return fileToArray(config.mergedBranches);
	} catch(err) {
		throw err;
	}
}

/**
 * @return {Array} - Array of branches that are config.branchAge or older AND not merged
 */
function findNotMergedOldBranches() {
	let branchesToBeDeleted = [];
	let notMergedBranches = [];
	try {
		//after getting branches that are not merged, we will filter them down to 2 months or older branches
		notMergedBranches = findNotMergedBranches();

		branchesToBeDeleted = notMergedBranches.filter(branchName => branchIsOld(branchName))
	   
	    //save the list to file for safe keeping
		fs.writeFileSync(config.notMergedOldBranches, branchesToBeDeleted.join('\n'));

	    return branchesToBeDeleted;
	} catch(err) {
		throw err;
	}
}

/**
 * @param  {branches} - An array branch names
 */
function deleteBranches(branches) {

	//extra caution against deleting develop or master branches
	let parentBranches = ['develop', 'master'];
	let filteredBranches = branches.filter(branch => parentBranches.includes(branch) !== true );

	filteredBranches.forEach((branch) => {
		try {
			console.log(branch);
			// execSync(`git push origin --delete ${branch}`, {stdio: ['ignore', 'ignore', 'ignore']});
		} catch(err) {
			throw err;
		}
	});
}

function init() {
	try {
		execSync('git checkout develop', {stdio: ['ignore', 'ignore', 'ignore']});

		let mergedBranches = findMergedBranches();
		let notMergedOldBranches = findNotMergedOldBranches();

		deleteBranches(mergedBranches);
		deleteBranches(notMergedOldBranches);
	} catch(err) {
		console.log(err);
	}
}

// run the program
init();

Updated to reflect input from James Benner which made the code more readable.

Add code to make the script checkout develop branch first.