While some would argue that the shift from 2D to 3D was the most significant moment in the history of Konami's Castlevania series, for others it was the release of Symphony of the Night in 1997.
This was the dawn of the "Metroidvania" genre – or at least it being recognised as a genre – and the game spawned multiple sequels on the GBA and DS, as well as countless imitators over the subsequent years.
Despite the success of SotN, there are many old-school Castlevania fans who prefer the pre-1997 template for the series – unofficially known as "Classicvania" – and a new fan-made title aims to appease those same people while marrying the NES-era titles with a more modern Metroidvania feel.
Lv4 Games has created Castlevania ReVamped (thanks, Pixel Cherry Ninja), a Windows and Linux-based title which boasts the look of the 8-bit Castlevania games but features the non-linear, gear-gated gameplay of the Metroidvania genre. It's an impressive mix, and one which arguably looks just as polished as Konami's own efforts in this field.
If you fancy checking it out, it's worth noting that the Linux version will run on Steam Deck. Download links are here.