ok I asked and the reason it's currently not part of the OSS version is that we want to keep Orion lightweight and providing email notifications that work for all sorts of email clients is challenging and might require maintaining unnecessary dependencies (and potential dependency conflicts) for users - I added an issue internally to investigate whether/how we may provide that as Prefect Collections with blocks
if any of that will be released, it will be announced in #CKNSX5WG3 and on discourse - in case you want to stay up to date