There was a recent case on SO where a user that was highly active in a very specialized tag asked on Meta.SO why their comment was deleted. The comment was linking to a site (non-profit, no ads) created by that user and to one of their SO answers. A few meta users essentially called this user a spammer, because any time you link to your own site and don't disclose it that is spam.
This kind of thing happens regularly in different variants. Users tend to stumble over the self-promotion rules, either because they simply didn't know them and are well-intentioned or because they are actually trying to self-promote. And I think far too many users are far too harsh and aggressive in these cases. I find some of the comments in the meta post very hostile and insulting, and those comments are heavily upvoted. Of course there are some users that self-promote excessively, but the distinguishing feature there is also that it is a pattern of behaviour. In this case it is about a single comment.
In this particular case, I'm familiar with the topic and the website the user linked to, and this only makes it much worse. For this particular, topic the website is one of the best resources on the internet, it is very far from some random blog spam.
There is one aspect I want to exclude for this discussion here, and that is the comment deletion itself because the issue there is not the self-promotion rule alone but also the discrepancy between how comments were designed and intended to use and how people actually use them, which is a giant topic on its own.
I want to focus on what happened on meta here, because while this case is a particularly bad case, it does represent a larger issue in my opinion. Some parts of the community react unnecessarily hostile when enforcing or explaining this rule. Calling anything that contains a link with undisclosed affiliation spam is hostile, insulting and also just plain wrong. Spam gets nuked and the accounts destroyed, in cases of self-promotion usually the rules are simply explained to the user and the links are fixed. Self-promotion is a somewhat related topic to spam, but it is also fundamentally different. Conflating the two issues leads to bad moderator actions and to overly hostile interactions with the often well-intentioned users.
I think we should give users that don't disclose affiliation to non-profit sites far more benefit of the doubt than they get right now. There is not that much harm to this unless it is truly excessive or untargeted. And we certainly shouldn't call users spammers (or their content spam) for breaking this rule. Pointing it out to users nicely should be enough in most cases, and I'd go as far as ignoring this entirely in many cases when the linked resources are well-known and respected in the particular topic.
I'm not sure if any specific rule change is necessary, though it might be helpful. The bigger issue is the mentality around this that has developed among some parts of the community, which is much harder to change than the rules themselves. But I do think that the overly rigid and harsh application of this rule is making Stack Exchange sites a less welcoming place.