Bear with me, I’m gonna argue against the previous, now-lost ruling pretty hard here, because that ruling is not consistent with existing rules and also bad for game balance.
Firstly, Indestructible units can totally be hit by “destroy” effects. Indestructible does not mean “this ignores anything which would destroy it”, and any claim pretending that Indestructible works that way is not based on sound reasoning.
So let me reference the actual rules:
I literally cannot see how it is possible to parse the interaction between those as anything but "Obliterate effects the entire set of {X lowest tech units} at once by destroying them, then Indestructible’s “if this would” clause triggers overwriting the destroy effect with an exhaust effect.
For gameplay balance reasons, I personally support interpreting “gone” in the Obliterate text to mean that if the exhaust overwrite from Obliterate sidelines an indestructible patroller, that makes it “gone” from the patrol zone and the the attacking unit can select a new thing to attack after the sidelining happens as per the Obliterate text - but that part is a highly subjective interpretation.
It’s also obvious that the “You can’t sacrifice this” clause in Indestructible means that Indestructible units are unaffected by Sacrifice the Weak, at which point a ruling is needed as to whether Sacrifice the Weak skips Indestructible units or fizzles and does nothing in the case where it would hit an Indestructible unit. In either case, a clarification is needed in database of specific card rulings - and until such a clarification is placed there it does not exist and is just the best guess of random strangers on the internet.
The potentially problematic interaction is between Garth’s Ultimate Death Rites and Indestructible, where an opponent’s units are destroyed one at a time and not in a set of X at once – meaning that a single low-tech indestructible unit such as Hardened Mox or an inactive Gargoyle turns an Ultimate spell into a “just exhaust a weak unit” effect. But I personally find that to be less of a game balance issue than the issue of a Patrol zone with Immortal in Squad Leader backed up by Heroes being immune to attacking Tech III units with Obliterate. It’s not like the game doesn’t already include many ways to render various other ultimates very weak - by denying opponents maxband heroes, casting free Speech, having armies that are untargettable due to Nebula or Lord of Shadows, placing flagbearers carefully, etc