Summarize, or state your position.

Is it a good idea to post only links in communication tools such as Slack or Discord?

There are several possible intentions of senders who post only links.
One is a strong request to read the full text (i.e., it makes no sense if you don't read the full text).
Second, it is tedious to clearly state an opinion or summarize information.
Third, trust in the other party that simply posting a link will convey all of one's intentions.

However, I see two problems with this.

One is that the sender's intentions are invisible.
We don't know whether the sender agrees or disagrees with the information in the link.
Or does the sender feel that the linked information is very important and wants to share it as knowledge within the community?
It is impossible to understand the sender's position on the information.

Second, it is inefficient in communicating information.
As mentioned above, since we do not know the intent of the sender, we must open the link without any preparation and interpret what kind of information it is. If the posting contains an outline, we can get an overview of the information in advance and then make a judgment as to what kind of information it is.
On the other hand, accessing information without preconceived notions may sometimes be a good idea.

Conclusion.

Certainly in a mature enough community, "what goes without saying" is shared by the members, and even a link-only post may perfectly convey the intent. It would be less costly that way.

I believe that information dissemination is for the benefit of the recipient. While it is possible for the sender to have the significance of the contribution, the information should basically be 100% for the benefit of the receiver.

If you take the position that the dissemination of information is for the benefit of the receiver, then I suggest that you post a link to the site with a few words.

TL;DR FTW!