I do not think that the previous suggestion to make clear what is being asked is limiting to arbitrary use case.
The point is that there is a fundamental difference between two scenarios: one where links are never dereferenced (aka followed) by either the client or server of nextcloud, which is inherently secure, and one where they need to be dereferenced (followed, which I understand is your use case), which can be troublesome in terms of security, resource usage and timelessness of sync and needs thus to be handled with more care.
To mention a few potential issues:
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How if, deep in your data, you end up having a symbolic link to a place with private sensitive stuff of yours? You may get that because your data is music and someone, maliciously, gives you a directory of mp3 files and you do not notice that one of the file is not a file, but a link to ~/Private. If links are dereferenced, immediately you start sharing sensitive data with many users. If users can upload symbolic links, the implications can be even more serious.
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How if, deep in your data, you end up having a cycle of symbolic links: A -> B -> C -> D -> E -> A. Unless some (possibly costy) algorithm is added to the sync code, the latter may start looping on dereferencing this cycle rather than doing what it should do.
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How if your sync dir contains a link to a volume, as you say, and a file in the volume is changed. In many situations the sync software cannot be reliably notified of this change, and the syncing of the modified file may start with a significant delay.
This is not to say that your usage scenario is not interesting (even if I think that in some cases the possibility to sync multiple folders can substitute for your usage of symbolic links). It is to say that only users who know what they are doing should IMHO activate it. Dropbox itself (that only supports the scenario you are talking about) actively discourages relaying on it because it can end up in bad surprises for users who do not fully understand what they are doing.
This is why, IMHO, the two cases should be considered separately and, if both implemented, have independent and mutually exclusive ways to enable them.
As a final clarification, Nextcloud currently implements (but disables by default for security reasons and due to the hash mismatches) the scenario you are talking about, but does not implement the one (never dereference) that I am talking about.