Not for all time, but for the last few years I've kept track on grouvee.com. Source: almost 3 years ago

Again, I am tracking them at grouvee.com now, so that is solved, but discovering new ones that's my main problem, it is possible to do that on grouvee as well, but it's not very convenient. Source: about 3 years ago

So I now track my games at grouvee.com, but I don't know where to look up ALL new releases for PC. Source: about 3 years ago

TrueSteamAchievements has this (in Easy Achievements). Source: 6 months ago

It does happen perfected games get new achievements added. Maybe that's what happened to your missing game? It's a real drag to make a separate list to compare Steam's list to, but maybe it's worthwhile signing up for something like True Steam Achievements if it's important to you. Source: over 1 year ago

OK looks like they've been picked up by truesteamachievements.com and some people have them unlocked. Source: over 1 year ago

A other way would also be to go on: https://truesteamachievements.com/. Source: over 1 year ago

Whether 100%'ing a specific game is worth it or not depends entirely on your preferences. I wouldn't put too much focus on achievements unless it brings you joy. I can't give you recommendations since I don't know your tastes. I'd simply pick a game you like. This website can help you track your progress, and it also has guides for most achievements. Source: over 1 year ago