Has there ever been a 5–5 in MLS?

Yes, 1 time.

The only time: May 8, 2004, San Jose Earthquakes and New York Red Bulls drew (San Jose Earthquakes 5–5 New York Red Bulls).

Nearby scores

1. 6–4 4 times
2. 6–5 Never
3. 6–6 Never
4. 5–4 13 times
5. 5–5 1 time
6. 4–4 18 times