What makes HarfBuzz 1.4 significant is that their "OpenType GX" branch was merged, thereby adding support for OpenType 1.8 Font Variations. I'm not too much into typography myself, but the OpenType GX / Font Variations support does appear to be a big step forward. Those wanting to learn more about it can see this video or medium post on the matter.
For the font variations there is also this useful overview from Microsoft.
OpenType Font Variations allow a font designer to incorporate multiple font faces within a font family into a single font resource. Variable fonts — fonts that use OpenType Font Variations mechanisms — provide great flexibility for content authors and designers while also allowing the font data to be represented in an efficient format. A variable font allows for continuous variation along some given design axis, such as weight...Conceptually, variable fonts define one or more axes over which design characteristics can vary. Weight is one possible axis of variation, but many different kinds of variation are possible. Variable fonts can combine two or more different axes of variation.
HarfBuzz 1.4 has this support, now making it much more widespread for the Linux desktop. Details on HarfBuzz 1.4.0 via Git.