See also also bike pic for other bike-related technical discussions and failed parts.
Rear derailleurs are commonly damaged by impact, but also commonly wear out when ridden in abrasive grit.
The Vivo Sport V1 derailleur used a conventional parallelogram but with a rubber boot to exclude abrasives and thus resist both clogging and abrasive wear of pivots leading to looseness.
More at Disraeli Gears' Vivo page.
Here is a used example (apparently used only lightly) with a black boot (plus a not-used red boot):
Here is the included instruction sheet:
And the box the used one came in:
Here is a not-used but shelf-worn example with a red boot:
Here is an example installed new, used intermittently, photographed after 7 years use but still in use after 12 years:
Slightly older front derailleur on the same bike same time:
Slightly older bottom bracket from the same bike, this unit is double-sealed with the normal bearing rubber seals plus an outboard O-ring on each side. Clearly, seals alone cannot exclude all stuff.