3080/90 are much more likely to thermal throttle because of the Memory Junction temperature than the GPU core.
Even watercooled cards (mainly 3090) can be frustrating as I was managing the GPU at nice mid 40's, and the Memory Junction would stay close to 80's, but no mining and not gaming at 4k here, so I wasn't pushing the memory hard.
The active backplate, despite some criticisms around, was a must for the 3090.
But to stay on air, quite few requirements:
Airflow, GOOD Airflow.
If your GPU wasn't assembled poorly, simply using some ghetto mod heatsink on the back of the card, using some good thermal adhesive tape, should help to avoid thermal throttling.
Is still happening, the only solution is replacing the thermalpad.
People struggling to find the Thermalright ones, can go with the Gelid Ultimate or Xtreme (slightly worse than the Ultimate).
Even replacing the thermalpads, at least on the back of the card, heatsinks still helps.
Also, not every card/scenario, but vertical mounting it can help. Just make sure it is not too close to the side window, otherwise you got yourself a glass hob.