they didn’t!
How??
Now that Jooha doesn’t remember anything from their past , Dooshik sees this as a blessing and decides to write a new story with Jooha that is filled with happy memories! So no they’re not dead!
source: trust me (no i’m serious tho, i’m 99% sure that they didn’t die right after jooha regained his memories) also! as someone mentioned in a previous comment, the author came out w merch very recently which shows an extra panel that comes along with one of the merch and people are speculating it shows their life after he regained his memories. (in the product photos, the panel (the panel is on a piece of like card stock probably) is blurred out but you can make out two figures, jooha and dooshik, i think they’re kissing or embracing?)
Im crying bruh what does this mean dont tell me they died