How do you identify important games in the Opening Explorer?

I want to switch to the French for an important tournament in October. I've not played it before.

For this, I think it would be useful to analyse ~50 important games.

Is there a way to identify 50 important games using the opening explorer and lichess's master games database?

First you need to decide which variations of the French you're going to play.
3 Nc3 : 3...Bb4, 3...Nf6 or 3...dxe4
3 Nd2: 3...c5, 3...Nf6, 3...Nc6, 3...a6 or 3...dxe4
3 e5: 3...c5 4 c3 Nc6 5 Nf3 Bd7 or 5...Qb6
Less important lines: Exchange 3 exd5, Two Knights 2 Nf3 d5 3 Nc3
For each of these choices there will be further subvariations

Then go to a database and look through recent games in your lines. They don't have to be 'important', but you should go for GM v GM games and quickly play through a bunch to see the typical strategies for Black. Then go back and look at non GM games as it's good to know how to punish some suboptimal White moves that are popular at lower levels.

@Realraptor said ^

important tournament in October

The new theory needs to be studied half a year before the tournament. And not in a month.

October is enough time to prepare. Once you've chosen your lines, play them in casual games at a fast time control. Afterwards, go through the games with an engine.