I know that some Stack Exchange sites (e.g. Writing) have community ads that they submit on other sites (example below). Is there a list of all such SE sites? I assume based on past questions that there isn't a list of all community ads. I am looking for a list (not necessarily comprehensive), of all SE sites with these promotional community ads. Quoting @Catija "sites that create community ads for themselves and post them on other sites' metas."
I originally understood your question backwards, but here is a SEDE query which tries to extract the links to sites. Only direct links to sites count, like this one in the example you mentioned. There are currently 27 sites doing this, some on multiple sister sites:
To make sure all links are considered (sometimes, they use a trailing slash, sometimes not; some links are still HTTP instead of HTTPS), I've wrote another SEDE query which lists all links and another one for the few ads which don't satisfy the format.
SEDE is updated once a week on Sunday morning, but that should be sufficient for this specific purpose.
You can easily get a list of all community ad answers that link to Stack Exchange sites via several global searches:
url:stackexchange.com [community-ads] is:a
url:stackoverflow.com [community-ads] is:a
- etc., with other domains.
You can cut down the number of results shown in these searches by adding
title:2019, which will only return the current ads or
score:6.., which will only return the ads which have enough score to actually be shown as ads.