It is a sin to participate in an indult Mass. Objectively yes.
Because
1. It is a danger to your Faith
2. It is a signal to others that you are participating in the destruction of the Church.
Now, if you were hanging out there afterwards, and had some very good reason, then I would say its probably ok. Maybe looking for a wife? Who knows.
If you were there for no reason at all, then maybe just leave it alone. Too much risk you will end up going there.