Let's get right to it. Open-chain exercises are not dangerous. Furthermore, knee extension exercises are not dangerous. They are not detrimental to any knee rehabilitation, whether it be a tendinopathy or ACL rehabilitation.
Similar to the "text neck" fear-mongering of increased stress, using relative terms fails to account for absolute forces. Just as "poor" posture causes increased forces and subsequent stress on the spine, open-chain exercises create increased stress at the patellar tendon. But the level of stress does not cause harm.
Regarding the ACL, the stresses are minor. In fact, walking creates more strain on the ACL graft than open-chain exercises. Studies assessing open chain exercises a mere 4 weeks post-op demonstrate no change in tensile strength or laxity.
The argument that open chain is unnecessary because closed chain exercises are superior fails to hold water. Open-chain exercises often elicit greater EMG activity and allow for the isolation of muscle groups. Closed-chain exercises allow for compensatory action and emphasis on other muscle groups. A knee extension exercise will isolate and rapidly hypertrophy the quadriceps - vital for ACL rehabilitation - while the squat heavily emphasizes gluteals and lumbar paraspinals (if using a barbell). Squats are immensely valuable, as are open-chain exercises. Furthermore, all exercises do not need to replicate a single "functional" movement. Controlling a limp in space, rapidly producing force, and building muscle all contribute to function.
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