Off the top of my head:
and
Otherwise Pubmed is (near enough) the equivalent of google for (English language) medical journals; so here's a link with some basic search terms:
rugby tackle height concussion - Search Results - PubMed
pubmed.ncbi.nlm.nih.gov
Of course, not all trials get written up in medical journals; or even in English
ETA: Ross Tucker's Patreon piece, previously referenced, contains this paragraph on currently red-card tackles versus non red-card tackles - so blunt tool
Obviously, he was writing a blog post, not an academic paper; and I don't see a reference for it, but I also see no reason to disbelieve him.