I would draw the line at “Does the post provide freely accessable, usable, non spam content?” If it’s just a link to to a store, or a unusability tiny sample of content then it’s an ad. But someone just posting unobtrusivly watermarked art or homebrew along with a donation link is fine and benefits the community. (This seems to mostly align with that)
Some subreddits had truly ridiculous politicies, like removing even very high effort, high quality content if is so much as mentioned the existence of a donation link. (Like r/Minecraft)
Other subreddits had excessively loose rules and got flooded with the same thumbnail sized artwork asking for commissions or advertising paid versions.