or even something hosted in person with actual demos like E3. Does anything like this exist?

The GamesCom in Germany always hosts the Indie Arena Booth, which has tons of indie devs densely packed in tiny booths, where you can play demos.

Since the pandemic, they’ve also had an online offering, where during the GamesCom, you can explore a virtual space with virtual booths, which will then also link to playable demos that you can download from Steam.