I assume the reason for the plugin type to be listed as legacy is because the version 5 symbol library format is still the old pre version 4 file format (not yet based on s-expressions).
Originally it was planned to get the new file format with version 5 but there were other things deemed more important so it was moved to version 6. It only just got introduced to the pre version 6 dev snapshots, so we are at least on track to getting the new file format for version 6.
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