What are you talking about that they did 'not offer Varnish for Buster?' Was there a packaging problem or more context you could add here?
Official Varnish repositorie (https://packagecloud.io/varnishcache) doesn't offer any release for Buster yet. 6.3 was added since 2 days, but not for Buster yet.
It looks like Debian 10 (Buster), which was released 2019-07-06 contains Varnish 6.1 instead of 6.2, which was released 2019-03-15.
6.3.0 is already out...
Since 2 days yes, but not for Buster.
Its opensource and maintained by volunteers. If this matters to you, don't whine on HN. Write a PR.