Should we (at least temporarily, e.g. some days or weeks before they occurs) promote FOSS community events (e.g. such as the current https://fosdem.org/2021/schedule/track/geospatial/) in a certain way, somehere on "our" Stack Exchange space?
For an event which is only days or weeks away, or already running, you could advertise it in the GIS Chat Room.
With a longer lead time the way to do it would be by taking advantage of Community Ads. These are the recent past ones:
and, even though Will there be Community Promotion Ads in 2021, if so when? is not yet answered by SE staff, I am expecting that the 2021 Q&A will appear on our Meta soon. In the meantime, in a comment, @Catija has said that:
until the new post is created, the old ones still give the required info to the system.