Hollow Knight: Lifeblood Update is currently in Beta on Steam (sadly Windows only) and it adds some rather nice stuff to the Metroidvania.

Here's what it adds in (full details here):

One boss has been seriously upgraded. A true challenge awaits!

A brand new boss with a new music track by Chris Larkin!

Map Markers are here! Cartographers rejoice!

Extras added to the Extras menu!

Numerous game balances and tweaks (Heavy Blow is now good... just kidding)

Various bug fixes

Additional sound effects and voices

Optimisations and performance enhancements

As said above, the Beta update is currently only for Windows and it's Steam-only (GOG doesn't really deal with Beta versions like this), with the full update expected sometime in the next 2 weeks for Linux, Mac and Windows.

Hollow Knight has seen some fantastic post-release support and content updates, I'm really pleased to see them continue developing it and upgrading it. I'm sure the map marker system will please many, as it's something I've seen people complain about.

The Linux version works great, available on GOG (affiliate link) and Steam.