Is it possible do disable users to delete tags?
If one user accidentally deletes tag (not only remove it from file) and other users use this tag too, this tag is removed globally so other users lose their tags so.
Is it possible to allow only admin to delete tags?
Hi Quba!
I'm also fighting with that issue right now 
Could you find any solution to avoid tag deleting?
Hi,
currently this is not a feature covered by ownCloud, but you can open a Pull Request or a Feature Request on github 
I have found dirty workaround (I am using nextcloud now, but it may be the same)
In the file
/lib/private/SystemTag/SystemTagManager.php
find function
deleteTags
comment lines below
// delete relations first
till the end of function.
This workaround prohibits deletion of tag in the database (tag seems to be deleted in user interface, but is not).
Pretty tricky
but as long as it works for you, it's fine with me.
You have to repeat this after an upgrade I would imagine
Yes, upgrade removes this workaround.