I recently started tracking the percentages for the results of the different rune chests that are now in the game, so hopefully people can get some kind of idea what to expect. It still baffles me that this information isn't made public, so we'll have to crowdsource that information ourselves. I'll share what I have here, and I'll try to keep it updated on a regular basis.
Level 2-4 Rune chest (266 total chests)
- Level 2 Rune: 81 (30.5%)
- Level 3 Rune: 97 (36.5%)
- Level 4 Rune: 88 (33.1%)
Level 3-5 Rune chest (168 total chests)
- Level 3 Rune: 165 (98.2%)
- Level 4 Rune: 1 (0.6%)
- Level 5 Rune: 2 (1.2%)
Level 4-6 Rune chest (122 total chests)
- Level 4 Rune: 120 (98.4%)
- Level 5 Rune: 1 (0.8%)
- Level 6 Rune: 1 (0.8%)
Level 5-X Rune chest (43 total chests)
- Level 5 Rune: 43 (100%)
- Level 6-X Rune: 0 (0%)
Level 7-X Rune chest (From PVP - 11 chests)
- Level 7 Rune: 9 (81.8%)
- Level 8 Rune: 1 (9.1%)
- Level 9 Rune: 1 (9.1%)
- Level X Rune: 0 (0%)
EDIT: Updated on 8/21/2019 with additional data.