i think the core requirement doesn't make much sense when you can unlock it with blue hearts. i would assume its there so that it knows the player has played gameplay harder than the a-sides, but with blue hearts being valid, it can instead ask if the player has solved a few puzzles. whether i think having it be locked behind 4 b-side completions would make sense, i have no idea because i completely lost touch with the difficulty curve of that part of the vanilla game years ago.

"farewell is cheap with all these mandatory pixel perfect moves" i can assure you theres nothing in farewell that is "pixel perfect". this game has tons of leniency measures to make sure there isn't, you just haven't noticed them yet. the intended shortest timings are 3-frames (1/20th second) too.

as for the farewell heart gate i think its completely fair. farewell isn't intended to be just the 9th chapter that you play right after core, its intended to be the end of the extra content in the game, being after b-sides and c-sides. you can still play it before c-sides but you're not really supposed to. everything past the heart gate is also post b-side difficulty.

The core heart gate is a bit silly, as core isn't unreasonably difficult. I'm assuming it made sense at the point when core was added to the game, as the story ends with chapter 7.

The farewell gate is reasonable, for the sole reason that the game is assuming you know movement tech from the B/C sides, that you will likely want to know about before attempting farewell.

FWIW, the heart gate for Farewell isn't a halfway point: the majority of the chapter, including some obscene difficulty spikes and a new movement tech, are after that gate. Farewell's difficulty level is notably higher than all of the B/C sides, so it's a bit silly to want to play it before you play (at least some of) the B/C sides.

Four hearts isn't much.

As for Farewell, it's notably harder than the C-Sides, so it makes sense to have you finish them all first before taking on the game's final challenge.

1. getting 4 hearts is super easy
2. farewell is far from pixel perfect
3. you can easily skip the heart requirement in farewell with 1 wavedash

If you think the first part of farewell is pixel perfect, you definitely aren't ready for the rest of the level. That heart gate is a blessing.