Even if the official rules don’t explicitly give komi values for other board sizes, it should certainly be an odd integer on odd boards to stay in the same spirit. Given this, 7.0 is the most reasonable value for sizes 9 and up. It’s our current best guess for perfect (area scoring) komi on all these sizes. Especially on 9x9, it would be surprising if perfect komi turns out to be anything else than 7.
A komi of 5.5 is quite a large handicap in Black’s favor between high level players on 9x9.
Edit: Maybe “quite large” is an exaggeration… here are some KataGo winrates on 9x9 (source) for reference:
| Komi | Black winrate |
|---|---|
| 5.0 | 83 |
| 5.5 | 69 |
| 6.0 | 69 |
| 6.5 | 70 |
| 7.0 | 41 |
| 7.5 | 13 |
5.5 - 6.5 are not terrible, but if we’re choosing between odd integers, 7.0 is a lot better than 5.0.